I had no idea as well but after seeing that definition I find it hilarious that Arcee, having perhaps a higher body count in these comics then... well, EVERYONE ELSE, would be the one chosen to bring this up. She shoots straight past abusive and into the realm of homicidal.
Actually, the 'abusive' is part of what the word means in a non personal context ('to slag someone off'). When directed at a woman, it's yet another term for 'promiscuous'.
From the Oxford English Dictionary
(e) A prostitute or promiscuous woman; a slattern.
1958 N.Z. Listener 10 Oct. 6/3 A ‘slag’ is a white girl who lives with or is friendly with coloured people of either sex.
1959 Streetwalker iv. 87 It's my pride he hurts, with his little slags.
1966 New Statesman 23 Dec. 934/2 You have to wear glasses these days if you don't want to be called a slag.
1970 ‘D. Craig’ Young Men may Die x. 72 Does anyone care what happens to a slag?
1973 J. Seabrook Loneliness 185, I went out with a girl called Angie, who was really a bit of a slag.