Perhaps from the notion that boots placed beneath one's bed knock one another due to the shaking of the bed during sex.

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knockboots (third-person singular simple presentknocks boots,present participleknocking boots,simple past and past participleknocked boots)

1. (slang,euphemistic) To havesexual intercourse, especiallyrecreationally if notcasually.
   • 1986, “Tramp”, inHot, Cool & Vicious, performed bySalt-n-Pepa:

     You ain'tknocking boots/ You ain't treating me like no prostitute

   • 1994,Marshall Crenshaw,Hollywood Rock, HarperPerennial, published1994,→ISBN, page250:

     Of course they fall madly and deeply in love, as Biker Dude learns to respect a girl who won'tknock boots on the first date.

   • 2016 February 14,Joseph Adenuga,Darren Dixon,Tariq Devega, “Ladies Hit Squad” (track 6), inKonnichiwa, performed bySkepta ft.D Double E andASAP Nast,Boy Better Know:

     After we done, bill a spliff and cotch / Pour me a glass of the Henny on the rocks / And get ready for round two / Cause any time weknock boots you know we knock twice

• do it,get some,hook up,Netflix and chill;bump nasties,slap bellies; see alsoThesaurus:copulate