Chastity Patch
A small smear of excrement surreptitiously smeared on the back of one's girlfiend's neck before a girls' night out, in order to deter any would-be suitors.
The missus was going out on her work Christmas party the other night and that smarmy git from accounts was there, so I gave her a Chastity Patch when she asked me to fasten her necklace.