A positive phrase, used as a replacement for "Fuck yeah!" when appropriate.

It's also a reference to the character Nick from the videogame Left 4 Dead 2, whose most popular phrase is (arguably) "TITS!"
Tadin: "Do you wanna go watch the Rocky Horror Picture Show?"
Cecil: "TITS YEAH!" *high five*
by SiempreSoloAnon March 22, 2011
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