(v) To have sexual intercourse. The knock refers to the man's hips hitting the woman's ass, which produces a slapping sound. The motion literally knocks her boots (buttocks).

fucking

doing the nasty

Knockin' boots is an old western term for having sex. Derived from both he and she stripping down and slipping their boots under the bed which sometimes made a "knocking" noise. Sometimes the boots are/were left on during sex and the boots would knock during all the action.