A regular shirt that has had both the sides cut off using scissors. Looks like a poor quality muscle-T.
Originated in Sonoma County, CA and has spread throughout the county high schools. Mainly wore by the jocks; football & basketball players.
Joe: I feel like making a new cut-off shirt today.
Devon: Another? Didn't you make like 2 last night?
Joe: Yeah, but I could use some new ones.