The chin was an area on one's face where the two sides of the jaw came together.
In 2151, while taking a photograph of Jonathan Archer, Trip Tucker asked him to tilt his chin up. (ENT: "Rogue Planet")
In 2366, Geordi La Forge got a left hook to the chin while the crew of the USS Enterprise-D was experiencing the side effects of Sarek's Bendii Syndrome. (TNG: "Sarek")
In 2376, The Doctor said he understood Lewis Zimmerman's motivations for creating his program in his own likeness, saying "Compassionate eyes and a strong chin can go a long way". (VOY: "Life Line")