The phrase girls will be girls is used to express resignation regarding an undesirable aspect of the behaviour of a girl or young woman, as being supposedly characteristic of her age or sex.
—Cf. also the phrase boys will be boys.
These are, in chronological order, the earliest occurrences of the phrase girls will be girls that I have found:
1-: From Hearts of Oak (London: Printed for James Ridgway, 1804), a comedy by the English playwright John Till Allingham (c. 1776-1812)—Ardent is Fanny’s father and Joe’s uncle:
Ard. What’s all this about?
Fan. Pray forgive me, I never will do so again—never—never—
Ard. Never do what?
Joe. Nothing at all, she has only had a bit of a sweetheart. Girls will be girls, uncle.
2-: From the Salem Gazette (Salem, Massachusetts, USA) of Friday 28th September 1810:
For the Salem Gazette.
The Monitress—No. XXXV.
Poetic numbers, aided by the power of music, have an almost irresistible controul [sic] over human passions, and they may be made the vehicle of the noblest, or of the most degrading sentiments. Is it not a deplorable fact, that they are too often made the vehicle of the latter? Such strength of mind, such stubborn virtue, if I may so speak, is requisite to enable a young woman of a pleasing exterior to act her part on the stage of life with the dignity befitting the christian [sic] character, that what to the superficial observer may appear like prudish affectation, or a refinement ridiculously overstrained, will strike the reflecting mind, of paramount importance.
The superficial observer will say, This is making a deal of noise about nothing; let the young people make themselves merry, and sing their love ditties; it is all natural, and suited to their age;—girls will be girls, and think more of sweet hearts than anything else.
3-: From Dudley (London: Printed by Strahan and Spottiswoode for Longman, Hurst, Rees, Orme, and Brown, 1819), an epistolary novel by the Irish poet and novelist Adelaide O’Keeffe (1776-1865):
Oh how I did, that very same evening, wish for a delicious gossip with an old friend! How I did recall our maiden days of confab, when sitting at a fire, or without a fire, until break of day, tongues active, spirits fagged, limbs cramped with cold, or scorched with flames, nerves in tremor at the slightest noise, (the rest of the family being all in bed,) a gust of wind through a key-hole, a creaking window-shutter, a piece of mortar dropped by a spider in his travels on the ceiling, a mouse, or rat, setting out on a foraging journey—then we start; all conjecture, all doubt, mystery, suggestion and debate; and then? Why then comes the wedding-day, and adieu to female confidantes; for, as I have often heard from our dear old licentiate M. D., “Girls will be girls: it is the nature of the sex to have their secrets; and let them: but once married, there must be no confidante but the husband. Rely upon it, there is more mischief caused by female friendship among married folks, than you young hoydens are aware of.”
4-: From Madeline, A Tale (Boston: Printed for Longman, Hurst, Rees, Orme, and Brown, 1822), by the English novelist Amelia Opie (née Alderson – 1769-1853):
“Bessie,” said my mother, ashamed of her vulgar forwardness, “you have not done the task I set you, it must be finished to-day: go and do it now;” and with a saucy toss of her head she bounced out of the room. “I cannot think where that girl gets her forward, pert manners,” said Richard. “I wonder,” observed Mr. Falconer, “that she has not profited by the examples she has had before her. Would she more resembled [sic] her amiable and gentle mother!” he added, kindly taking my mother’s hand, and looking with smiling approbation in her still handsome face. Tears of pleasure rushed into Richard’s eyes at this compliment to his mother, and he looked as if he wished to give the laird one of his energetic hugs: but respect forbade. My mother was not so well pleased; for the satisfaction she felt at hearing her own praises from Glencarron, was overbalanced by the pain of hearing his censure of her child, and with a truly maternal feeling, she said—“Nay, nay, Glencarron, pray recollect that Bessie is only sixteen, and girls will be girls. She will be more like her sisters in time.”
5-: From Granby (London: Henry Colburn, 1826), by the English novelist Thomas Henry Lister (1800-1842):
“Chesterton, though very desirable as a husband for those who are seeking for rank and fortune, is terribly heavy as a suitor—he brings up such a long battering train of clumsy, round-about speeches. He has none of your soft, sly, sentimental small shot. That is the attack the ladies prefer. They like to be pelted with sugar plums, as we used to do at Rome, in the Carnival—No—poor Chesterton!—ha-ha-ha—she really used him rather ill.”
“How so?” said Granby, as calmly as he could.
“Why, girls will be girls. They like admiration, as you and I know; and this Miss Jermyn liked a little more, or rather, liked it better served up, than Chesterton, poor fellow, had the means to afford.”
6-: From a letter to the Editor, published in The Globe and Traveller (London, England) of Thursday 28th November 1833:
Sir,—As most of your readers are liable to be chosen to serve the important office of overseer in their respective parishes, a few hints perhaps in these present times, on the old and most approved methods of discharging its duties, may not be unacceptable to many of them.
I am led principally to offer the suggestions that follow in consequence of the complaints of a neighbour lately chosen overseer of the parish in which I reside, who roundly declares he is but playing the fool in the parochial farce. He argues somewhat in this way:—“Though the Bible,” says he, “declares ‘man shall live by the sweat of his brow,’ the law says he need not; for if A won’t work, B must maintain him; if C be imprudent, D must supply him. Now if I,” continues he, “the overseer, being one of B and D, demur to this arrangement, the very beggar who is eating my bread may cite me to answer for my conduct before the bench, misnamed, of justices;” that thus, in fact, he is placed by his fellow parishioners in the front of the battle with his hands tied behind him. To such rebellious murmurs, then, should the following supply an answer, and manifest how easily to himself and satisfactorily to his neighbours his part may be performed, I shall be satisfied in having addressed you, if pardoned only for the space I may occupy in your valuable columns.
As two men are stronger than one, the strength of a nation must depend on the amount of its population. The overseer, therefore, will see that every man has 1s. 6d. per week for every child he gets—a just reward only for his gratuitous exertions in favour of the state.
Every girl that “happens an accident,” he will have supplied with proper baby linen, and whatever other accommodation she may stand in need of; she will afterwards be furnished with 2s. per week, at the least, provided she keeps the baby at home as a warning to her younger sisters. N. B. He is to recover the money from the father if he can—if not it cannot be helped; poor things! girls will he girls!
All the surplus children thus engendered, not otherwise provided for, he will take care are properly fed, and clothed in pretty uniform, and walked on Sunday two and two to church; such interesting sight cannot but act as an additional spur to population.