cuck

1) A gentleman. A man who accepts that they are not the master of their girlfriend's leisure time or social life. A man who takes his relationship on trust, and does not interpret friendly platonic interactions his girlfriend has with other men as flirting or a sign that she is cheating on him. A man who does not insist that his girlfriend come to him for his approval for how she spends her time or who she spends it with.
A man who accepts that his girlfriend may have a romantic and sexual past, and does not automatically treat her exes as rivals or insist she cut all ties with them if they are still friends. A man who accepts that his girlfriend may have a romantic and sexual future if their relationship fails and she moves on.
A man who does not routinely pressure his girlfriend into sex, particularly sex which is not also gratifying for her or for sex she has already said she is not comfortable with (i.e. sex without a condom, anal sex, a threesome with her hot friend).
A man who is prepared to put his girlfriend's needs ahead of his wants.

2) A committed, affectionate boyfriend. Someone who likes to treat his girlfriend well, get to know her friends and her family, keep up a date night tradition in a long-term relationship, put time and effort into choosing gifts she'll appreciate, gratify her sexually. A boyfriend who likes it when his girlfriend feels good.

3) Not a fuckboy.

Derived from cuckold, but with radically different meaning.

-See that... just letting his girl talk to that dude over there. Man, Steve is such a cuck.

- Mark bailed on us. Said he was spending tonight with his girlfriend or some BS like that. What a cuck.