After arousing your girlfriends
sexual interests, you forcibly shove half of your clenched fist up her ass (hence the "quarter", since a "
full" would be both fists), whilst simultaneously ejaculating into her
anus.
The quarter pounder with cheese:
Bob: Once I got Harriet in the
mood, I gave her the ol' "quarter pounder with cheese" to liven things up a little!
Doug:
Sick!