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.