To have sex. Usually describes a man having sex with a woman. Used in Northern ireland exclusivly.
"she's fuckin' powerful, I'd buck the hole of her."
"get 'er bucked, big lad!"
"Haha, we caught him bucking some ugly bitch!"
"aye, he's fuckin' devestated. he just found out that she was gettin' bucked by some squaddie while he was lookin' after the kids"