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)
