your somebody's knees are knocking 成语
the wolf knocking
(See the wolf is at the door)
knocking boots
to have sexual intercourse; the knock refers to the man's hips hitting the woman's ass, which produces a slapping sound. The motion litterally knocks her boots (buttocks): "Bring your friend, I likes you two's I'll knock the boots, the sneakers and the house shoes." -- Digital Underground (Flowing on the D-line [??])