Green’s Dictionary of Slang

1. to steal clothes that are hanging out to dry.

1853		Greenock Advertiser 28 June 4/2: They do not, however, restrict themselves entirely to ‘snow-dropping.’ They show great dexterity in stripping hen-roosts.
1949		R. Park Poor Man’s Orange 244: What about that blouse of mine that someone snowdropped off the line?
1989		J. Morton Lowspeak.
2001		I. Welsh Glue 35: Ye want clothes, ye snowdroap thum offay some cunt’s line.
2012		(con. 1980s) I. Welsh Skagboys 542: That sky-blue Fair Isle jumper [...] the one ye fuckin snowdropped fae the drying green at the Bannanay flats.
2018		I. Welsh Dead Man’s Trousers [33]: [T]he sentimental snowdropping gyppo cunt.

2. to rob someone, e.g. a drunkard, of their clothes.

1905

Sun. Times (Perth) 26 Mar. 1/1: On his way from the office someone snow-dropped him for his pants [and] as his cash was collared too he has sworn off swanky till next pay.