The act of having sex with a female, then after you cum you lay on top of her, shit on her stomach. Then you take your hand and slap her stomach has hard as you can. it is in regards to the imfamous comedian who used to smash fruit with hammers which then splattered all over the audience.
I shit all over that chick and then did a Gallagher all over her room.
Gallagher; verb ; When your girlfriend is giving you head, suddenly grab the back of her skull and jam your dick down her throat.. This is usually used when she has been being a bitch or just needs to be put in her place.
My girlfriend was complaining how our relationship is all about me, so while she was blowing me, i gallaghered the bitch.