Green’s Dictionary of Slang

blow off on the groundsills v.

also blow off on the groundsill, blow off the groundsills, blow the groundels, … groundsel, …groundsill
[SE blow + groundsills, the foundation or lowest part of any structure, i.e. one’s panting breath is exhaled at ground level]

(UK Und.) to have sexual intercourse while lying on the floor.

[UK]Head Canting Academy (2nd edn) 170: Blow off on the Groundsils O.Q.P. on the Floor or Stairs.
[UK]B.E. Dict. Canting Crew n.p.: Blow-off on the grounsills, c. to lie with a Woman on the Floor or Stairs.
[UK]J. Shirley Triumph of Wit 194: Blow off on the Groundsil. [Make away from the Stairs lest you are taken].
[UK]New Canting Dict. n.p.: blow off the grounsils i.e. To lie with a Woman on the Floor or Stairs.
[UK]Bailey Universal Etym. Eng. Dict.
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: to blow the groundsils, (cant) to lie with a woman on the floor.
[UK]Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: To blow the grounsils. To lie with a woman on the floor. Cant.
[UK]Lex. Balatronicum [as cit. 1796].
[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 12: ‘Blow the Groundsel,’ is when the pair lie on the floor awhile.
[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Farmer Vocabula Amatoria (1966) 79: coucher. To copulate; ‘to blow the groundels’.