(1) Wearing more than one condom during intercourse to protect against STDs that may be contracted from shady sexual partner(s). Would also apply if doubling up with other forms of protection from STDs.
(2) To go on the prowl with the specific intent of finding shady sexual partners.
"If you're gonna hook up with that skank, you better polybag that thing."
"Dude, I'm going polybagging tonight. Be my wing man?"