[A humble request: If you can, please donate to help me carry on tracing word histories. Thank you.]
The noun long trousers is used in reference to the wearing of long rather than short trousers as a mark of increasing maturity. In particular, the phrase not yet in long trousers, and its variants, mean: young, at an early stage of development.
The noun long trousers occurs, for example, in a review of Beethoven: Complete Violin Sonatas (Warner Classics, 2014), by the Japanese violinist Daishin Kashimoto (born 1979)—review published in The Observer (London, England) of Sunday 9th March 2014 [page 28, column 5]:
Daishin Kashimoto, a Japanese national born in London who studied at the Julliard when barely in long trousers, started prodigiously young—he gave his first full recital aged nine and has scooped most of the world’s top violin prizes.
—Cf. also the phrases to be all mouth and (no) trousers and anything in trousers.
These are, in chronological order, the earliest occurrences that I have found of the noun long trousers used of increasing maturity:
1-: From My Brother Jack. A Story for St. Valentine’s Day, a short story by ‘A. H. P.’, published in The Taunton Courier and Western Advertiser (Taunton, Somerset, England) of Wednesday 18th February 1874 [page 3, column 2]:
If there ever was a human being whom I envied in my life it was my Brother Jack. My earliest recollections point to the admiration with which I viewed him as it were afar off. He was the idol of my childhood and the hero of my youth, and, must I say it, the warning of my maturer years. I remember well how I envied Jack when he had his first long trousers and round jacket, while I was yet doomed to knickerbockers and a belted tunic.
2-: From the Burlington Daily Sentinel (Burlington, Vermont, USA) of Monday 25th January 1875 [page 2, column 2]:
—The happiest person in the world—A nine-years’ old boy in his first long trousers with suspenders.
3-: From the following advertisement for Saks & Company, published in The Washington Critic (Washington, District of Columbia, USA) of Friday 2nd October 1885 [page 4, column 3]:
Friday and Saturday is always a great day in our Boys’ and Children’s Department. All day long the little fellows, accompanied by mother or sister, may be seen examining with critical eye, worthy of so important a matter, the different goods placed before them. The little man, who is for the first time dressed in long trousers, looks as if he felt the importance of the fact that he has taken one step nearer to manhood, and if it be true that “The child is father to the man,” we can only wish that our little “father” may have many occasions in years to come that will give him as much happiness as his first long trousers.
4-: From the following advertisement for M. Meyer & Co., published in The Morning Times (Selma, Alabama, USA) of Tuesday 6th October 1885 [page 4, column 6]:
“YOUNG MAN,”
Let us have a short chat together on the subject of MEN’S APPAREL. You are the boy that visited us years ago, with your mother, and gloried in the possession of your first long trousers. Years have passed since then. You are now a stylish young man. [&c.]
5-: From The New-York Times (New York City, New York, USA) of Friday 8th April 1887 [page 4, column 7]:
DRILLING LIKE VETERANS.
THE BERKELEY SCHOOL BOYS GIVE THE MILITIA POINTS.The circle of seats around the floor of the Seventh Regiment Armory and the balconies were filled last evening with a large and appreciative gathering who had come to see the boys of the Berkeley School drill and perform evolutions that make soldiers. It was a pretty drill, and the boys, nearly whole companies of whom are not yet in long trousers, marched with the steadiness of veterans.
6-: From the column All Around Town, published in The Buffalo Daily Courier (Buffalo, New York, USA) of Friday 23rd September 1887 [page 4, column 2]:
—“Alas and alas,” sighed an old man on the curbstone in Main street yesterday morning as the procession went by, “I wish I could be a boy again two days in the year,” and he gazed mournfully at the clown.
“What days are those?” inquired a sympathizing friend.
“Fo’th o’ July and circus day. I’ve never known a moment’s perfect happiness since I was twelve years old, donned my first long trousers, and resolved to be a small boy no longer. Since then I’ve been an alderman, partaken of champagne suppers, owned a fast horse and a stock farm, but none of these have ever filled me with the same intoxication as sitting on a seat without any back under the circus tent, or firing off a toy cannon on the morning of Independence day. Life is all an empty dream now.”
7-: From a transcript of the address on dress that one Mr T. T. Wyly Armstrong delivered on Monday 21st October 1889 at the weekly meeting of the Newry Church of Ireland Young Men’s Society—transcript published in The Newry Telegraph (Newry, County Down, Ireland) of Thursday 24th October 1889 [page 4, column 5]:
Look at a boy’s delight when first he is permitted to cast aside the petticoats of a girl for knickerbockers. These in turn are consigned to oblivion in the twinkling of an eye when he gets his first long trousers.
word histories 