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1.
(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).
"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)
by Stoic Seraph April 14, 2003
 
2.
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.
I think I got drip dick from knocking boots with that hussie Ellie May.
by Bill Dailey September 22, 2006
 
3.
fucking
i was knocking boots with your mom
by Sean October 15, 2003
 
5.
The act of knocking two boots together, usually leather, for cleaning purposes or to test the strength
Why are you knocking boots?

They have mud on them.
by F. Roy. Dean Schlipps October 09, 2011