Green’s Dictionary of Slang

an up-market whore, kept by a rich gallant; also used as an endearment.

1593		Holyband Dict. n.p.: Tu es un cainar, thou art a squall [F&H].
1604		Middleton Father Hubburd’s Tales line 583: They should be none of these common Molls neither, but discontented and unfortunate gentlewomen [...] and they, poor squalls, with a little money, which cannot hold out long without some comings in; but they will rather venture a maidenhead than want a head-tire; such shuttlecocks as these, which, though they are tossed and played withal, go still like maids all white on the top.
1607		Middleton Michaelmas Term III i: Who would think now this fine sophisticated squall came out of the bosom of a barn and the loins of a hay-tosser?
1611		R. Cotgrave Dict. of Fr. and Eng. Tongues n.p.: Obeseau [...] a young minx, or little proud squall.
1630		J. Taylor ‘A Whore’ in Works (1869) II 112: The rich Gull Gallant calls her Deare and Loue, Ducke, Lambe, Squall, Sweetheart, Cony, and his Doue.