In LGBT+ context, to slay means to do/give something really well, to the point it impresses lots of
people around you; which is comparable to (but not fully definitive of) being a soldier on a battlefield slaying everyone around him in
high precision. The word 'slay' was popularised especially by the drag community, with many using it at ballroom events to complement drag queens who expressed themselves with sharp, flattering feminine energy.
For a very long time, 'slay' was a word especially centred to the LGBT+ community, especially for drag queens (the word found more prominence on RuPaul's Drag Race), and femme gay
men; therefore, it was a word only
people around a certain community knew of.
And then, along came Beyoncé (with especially the song Formation), causing the word to enter the vocabulary of many
straight girls for the same usage and purposes (and entered even worse popularity by
TikTok).
Before
2016:
Drag Race host: Girl, I gotta say, you absolutely slayed on that lip sync battle!
After
2016:
Straight teenage girl: Hey, I have pics of Sabrina at the red carpet.
Like, oh my god, have you seen her dress? It actually slays.