Green’s Dictionary of Slang

1. (orig. US black) an unattractive, slutty-looking woman.

1938		‘Sl. among Nebraska Negroes’ in AS XIII:4 Dec. 316/2: A young woman of none too pleasing appearance is a skag or a hag.
1944		Chillicothe (MO) Constitution-Trib. 11 Aug. 2/3: On your leave may there be [...] lots of skags, full moons and plenty of Coca Cola.
1964		E. Wilson Earl Wilson’s N.Y. 28: Scag – An unbelievably ugly girl.
1970		Current Sl. IV:2 9: Scag, n. An unattractive girl.
1983		S. King Christine 86: Howie, who was that skag your brother was wit?
1990		(con. early 1950s) J. Ellroy L.A. Confidential 160: Duke Cathcart had two skags in his stable [...] he was a pimp disaster area.
1992		D. Burke Street Talk 2.

2. a promiscuous woman.

1980		E. Folb Runnin’ Down Some Lines 145: Skag’s a tramp! One o’ dem disgraceful young ladies! Take anythang she can git – the leftovers! Go wid anybody.
2005		Mad mag. Apr. 10: Queen Amidala, that Hermione skag and the sister from ‘Spy Kids’ have all ‘hit the wall’.

3. a tease.

1980		E. Folb Runnin’ Down Some Lines 145: Skag who dey call a girl. Like if try to play hard to get.
1991		B. Hamper Rivethead (1992) 80: Useless lyin’ skag bait cunt.