Green’s Dictionary of Slang

(US campus/teen) a male’s back-handed compliment used to a woman in the hope of seduction; also as v.

2004		N.Y. Times 25 Jan. 🌐 The ‘neg,’ for example, is his [i.e. the ‘pick-up artist’ self-named ‘Mystery’] invention. Neither a compliment nor an insult, a neg holds two purposes: to momentarily lower a woman's self-esteem and to suggest an intriguing disinterest. ("Nice nails. Are they real? No? Oh, they look nice anyway.") .
2005		Urban Dict. 29 Nov. 🌐 A light insult wrapped in the package of a complement. Used by pick-up artists to gain and maintain the attention of women who possess uncommon beauty (9+). These women are immune to standard complement.
2016		C. Eble (ed.) UNC-CH Campus Sl. Spring 2016 6: NEG — seduce with a series of insulting, back-handed compliments.
2017		www.merriam-webster.com 28 June 🌐 Negging involves throwing a mild insult at a woman. The intention is to surprise her enough for her to take an interest in the confident and judgmental man issuing the insult (rather than assuming he is a creep).