Dress Code Violation?

Saw this t-shirt on a student the other day:  "My other t-shirt is on your mom".

When I read it I sort of cocked my head and raised an eyebrow.  Another kid came over and was making fun of the tshirt...not because he found it funny but because neither one of them had ANY idea what it meant.

Quandry - do I tell him to remove it and then have to explain to a clueless middle schooler what it meant or do I let it go and hope that no one else understands it either?  Let's just say I took the easy way out.