Probably from an infixed form of the root that gave rise to modern cứng (“hard”).
nứng • (, 𦣘)
- (vulgar, of genitals in general) sexually excited
- (vulgar, of a penis) (rock-)hard
- Synonym: cương
- (vulgar) sexually aroused; horny
Mày nứng à?- The fuck is wrong with you?
- (literally, “Are you horny?”)