The phrase to write home about is used predicatively to mean to boast of, to get excited about. It usually appears in negative contexts, especially in nothing to write home about, denoting something that is unremarkable or mediocre.

This phrase is based on the image of something that is worth writing to one’s friends or family at home about, as exemplified by the following passage from In a Terrible Scrape! — or, — The Perils of mixing in Good Society abroad, a short story by ‘A Sufferer!’, published in The Flag of Our Union (Boston, Massachusetts, USA) of Saturday 23^rd June 1860—while in Brussels, the narrator, Jack Bigsby, from the USA, has just encountered Count Paul del Caroville and his wife; Jack starts fantasising:

“Ah, Jack Bigsby!” I exclaimed, “you may think yourself a fortunate fellow, to have made such acquaintances as the count and his charming wife. It will be something to write home about. How some folks that I could mention will envy me my intimacy with people of such evident distinction! How well it sounds! I wonder if it will appear in the fashionable items of the New York Herald? Just fancy it.
“‘Brussels.—There are several Americans in this city. Among the rest we may mention John Bigsby, Esq, of 39^th Street, who has been a frequent guest of the Count and Countess Caroville. He has on more than one occasion attended the Countess del Caroville to the Opera Italienne, where the count has a box for the season.’”

(Jack Bigsby will eventually lose all his money, the Count and Countess turning out to be two swindlers, Jean Tortoni and Marguerite Guircolli.)

The earliest instance of nothing to write home about that I have found is from the account of a baseball match between the Stars and Y.M.C., published in The Daily Herald (Calgary, Alberta, Canada) of Thursday 13^th July 1905:

Irving, of Floto show* fame, was the contortionist for the Young Men, and pitched fair ball. Nothing to write home about, but it was certainly better than his support.

* Floto show is apparently a reference to the American circus originally named The Otto Floto’s Dog and Pony Show.

The phrase also appears as (not) worth writing home about. The earliest occurrence that I have found is from the review of “A Runaway Girl, seen last evening on our stage”, published in The Rockford Daily Register-Gazette (Rockford, Illinois, USA) of Saturday 16^th December 1899:

A Runaway Girl represents the climax of modern entertainment and good taste and it is cause for some self-satisfaction that this city gave it an audience which filled the theater to the doors.
“Trouble,” says Flipper in the first act. “No, never been there, my boy.” To a place on the opposite side of the earth the audience was taken by the jolly man from Cook’s. The guide spoke the language of the Land of Merry Jest and the itinerary included a two-hour tour which is worth writing home about.

The following is from an advertisement for Scott’s Toggery, 438 Westminster Avenue, Vancouver, published in The Vancouver Daily Province (Vancouver, British Columbia, Canada) of Saturday 2^nd March 1907:

A thing worth writing home about—was our shirt sale yesterday.
To-day there are still enough bargains left to create another furore.