Cuck is a man who's a little bitch. Contrary to the beliefs of the liberal-leaning crowd trying to explain something popularized by the conservatives, cuck is used by many races for someone who is spineless and IS derived from cuckold.
Her cuck boyfriend watched as a man flirted and felt up his girl in front of him.

She cheated on him and told him it would never happen again, he's such a cuck to believe that.