Green’s Dictionary of Slang

rub-a-dub(-dub) n.

[SE rub-a-dub, the beat of a drum]

(orig. US) sexual intercourse, esp. quick and spontaneous.

[US]Southern & Hoffenberg Candy (1970) 100: You get your rubadub, don’t you lady?
[UK] ‘As I Was Riding’ in Bold (1979) 10: And they were having a rub-a-dub-dub / Right before his eyes.
[UK]T. Blacker Fixx 39: Every rub-a-dub-dub, every hey-diddle-diddle.
[UK]Solar Project ‘Zeitgest’ 🎵 on ...in Time [album] We honk for cookies, we knock it out / We mash the fat, we mess around / Jig-a-jig, rub-a-dub, dead shot, Donald Duck / You wanna feel my Bethlehem steel / Mary Poppins, TNT, Bristol City / I bite into your cats and kitties.
[UK](con. 1981) A. Wheatle East of Acre Lane 17: Did you get any rub-a-dub at the party?

In compounds

rub-a-dub soldier (n.) [Carib. E. rub-a-dub, heavy instrumental music, also a type of dance]

(W.I.) a dancehall music enthusiast.

[US]Blood Fire Posse [song title] Rub A Dub Soldier.
[WI]Francis-Jackson Official Dancehall Dict. 44: Rub-a-dub soldier dancehall enthusiast.