A trisec was unit of time on the planet Triskelion.
In 2268, Galt gave James T. Kirk and Kloog a rest interval of fifteen trisecs during their fight. (TOS: "The Gamesters of Triskelion")
Fifteen trisecs was roughly equal to forty-five seconds, so it can be inferred that the term "trisec" is literal (if abbreviated) and, in fact, equaled three Earth seconds.