long in the tooth

Possibly from the practice of examining the length of horses’ teeth when estimating their ages: an old horse has long, rectangular incisors, and their occlusion angle is steep. Compare don't look a gift horse in the mouth.

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long in the tooth

1. (idiomatic) Old, aged.
   • 1852, William Makepeace Thackeray, The History of Henry Esmond, Esq., ch. 2,

     His cousin was now of more than middle age. . . . She was lean, and yellow, and long in the tooth.

   • 2004, Chris Taylor, "Is Microsoft A Slowpoke?," Time, 10 May,

     So as Microsoft began its 30th year last month, investors wondered whether it's a little long in the tooth.

• don't look a gift horse in the mouth