know (one's) shit(redirected from they knew their shit)
know (one's) shit
rude slang To be very knowledgeable, especially in a particular area. That piano tuner really knows his shit—you won't believe how good the ol' Steinway sounds now. A: "I don't know about running this play, it seems like the wrong move." B: "Don't worry, Coach knows his shit."
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.