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]

[late 17C– late 18C] (UK Und.) to have sexual intercourse while lying on the floor.