Literally a sexual boy toy who belongs to an older man. The younger guy is a bottom or anal receiver while the older guy is the pitcher, or top. Also, an extremely submissive and ass-kissing employee or student.
Who's the tasty twink over there by the bar? Oh that's the bartender's new butt boy.
Hey guys, I got an A+ in History. No wonder, you are such the teacher's butt boy!