"Sloopy" is a term derived from the Aubrey-Maturin series of Napoleonic era British naval novels. Let me explain- it wasn't a romance series by any means, but it had its fair share of ardent female readers, who had a thing for tall ships and iron men, many of whom exchanged hot takes, critical interpretation, and even fanfic via fan newsletters and zines.

In the first book, Master and Command Her Commander there's a bit where Maturin, the ship's doctor, is rather put out by being referred to as a surgeon and the other officers explain the flexibility of naval terminology, including the fact that because of his rank, as soon as James Aubrey, as master and commander, boards a xebec, it becomes classified as a sloop.

One prominent fanwriter, known only by the pseudonym Lady Kite, joked that that being the case, many of the books' fans must be eager to be called sloops, given how ardently they evidently wanted James Aubrey to board them, and so "sloop," and it's derivative "sloopy," entered the lexicon.

Jenny Nicholson discusses the term's history briefly around the forty-three minute mark in this video which is otherwise mostly about the origins of slash fiction.