Fly undone

'We just brought that guy in, the one hanging around the schools. You want to talk to him? You said you did.
'Sure. What'd you get him for?'
'Fly was undone.'
'He waive his right?'
'Yep. He has an explanation.'

What's 'fly was undone' mean?

One's "fly" is the part of your pants over the zipper. So if you say your fly is down or undone or whatever, it means your pants aren't zipped up.

Oh yeah, that's right. Dirty pervert.
Thanks Skippy.