Gin was such a popular drink in early Georgian Britain that, inevitably, it spawned a number of synonyms. Sky blue, Francis Grose informs us in his A Classical Dictionary of the Vulgar Tongue (1785) was one such. Doubtless, it was ideal for sluicing your gob, taking a hearty drink.
Intriguingly, there is an entry for sky farmers whom he defines as “cheats who pretend they were farmers in the isle of Sky (sic) or some other remote place, and were ruined by some flood, hurricane or some such public calamity”, Fraudulent beggars were a problem even then.
A slap-bang shop was a petty cook’s shop where no credit was given. All goods had to be paid for “with the ready slap-bang” aka immediately. It was also a slang term used to describe a stagecoach, a caravan or a night cellar frequented by thieves.
If you are looking for a synonym for a fool’s errand, try a sleeveless errand, “a search for something which is impossible”, while sleepy was a term used to describe something much worn, as in “the cloth of your coat must be extremely sleepy, for it has not had a nap this long time”.
We tend to describe a misuse of words as a malapropism after Mrs Malaprop, a character in Sheridan’s 1775 comedy of manners, The Rivals. However, in Grose’s time, or earlier, one who misnamed or misapplied a hard word was slipslopping, a reference to Mrs Slipslop who appeared in Henry Fielding’s 1745 novel, Joseph Andrews. I think I prefer the earlier term.