TTU is an acronym which stands for "Too Turned Up." When someone is turned up, it means that they are high or drunk off something. Being too turned up means that the individual is too high or drunk, which will likely lead them to doing regrettable things or stuff they wouldn't do normally.
TTU is often used in hip hop music, in Chris Brown's "Real One" song, he raps, "Double cuppin' I stay too turned up cause we poppin' bottles in here - Baby I'm just tryna party, yeah cause I'm the man of the year." In Yung Simmie's "Lean With The Sprite" song, he raps, "Blunt rolled tight, who got the lean - Too turned up, who do it like me."