Peter Gallagher

Verb. To "Peter Gallagher" someone means to engage in sexual relations with his or her spouse, life partner, or any sort of very close significant other.

This is taken from a very predominant theme in film--that Peter Gallagher (or any character played by Peter Gallagher in film or even stage drama) sleeps with the other guy's wife.

Syn: to cuckold, to give horns (to)

Iago to Othello:
"You mean that fuck Cassio has been Peter Gallaghering me the whole time?!"