sort of like bisexual, except it is when one likes males, females, its or shemales.
bob: "dude i sucked my girlfriends dick last night"
jack: "WHAT THE FU..."
bob: "-WHAT!? im trisexual
jack: "well i guess that explains the chick, dude, and it u slept with last week."
bob: "yup."
Casey M., a herm, keeps talking about how it wants to viciously molest Jamal, a black man, Dorothy, a white woman, and Tyrome Britney, a mexican hermphrodite. In this case, (s)he is considered a trisexual.
All the above definitions cover the concept "pansexual", i.e. willing to have sex with anything, be it man, beast, rock etc. My rather waywaerd friends believe it to mean "someone who beleives themselves to be homo-, hetero- or bi-sexual at any point in time, and believes it completely", for example someone who is gay one day, straight the next, bi on another day.
A combination of straight, gay, bisexual and all those linked together. A life form that will do "it" with almost anyone, anything at any time no preferences. Desperation.
Bob is nothing more than a trisexual. He'll practically do it with anything, anyone.