Killing it.
Your outfit slays everytime.
Originally meaning "to kill" or "slaughter, "slay" as a slang term has been used off an on throughout the years. In the 1920s, it was used to mean "to make someone laugh very hard." You slay me! It finally grew in prominence and use, however, in the 1970s and 80s in the ballroom scene. Ballroom was a then underground subculture that was a safe place for queer people of color to show styles of fashion, dance, artistry, and self expression. Phrases such as "yas," "werk," and "slay" were praise given to performers who showcased immense talent, showmanship, and commitment to pageantry of the ball scene. It was also in the ballroom scene that "slay" started to refer to the flawlessness of one's hair, makeup, clothing, attitude, and sense of style.
The lingo spread throughout gay culture and drag scene. It was featured in the 1991 documentary Paris is Burning and the television reality program, RuPaul's Drag Race in 2009 introduced TV audiences to the term "slay" and many more like it. The term became a more enduring phrase when Beyoncé in 2016 featured the word in her song Formation, where she told the girls to get in formation and slay or get eliminated.
As the term has now penetrated mainstream culture, you can hear "slay" in more and more settings outside of the queer space. It is now often heard in the context of fashion ("that outfits slays"), beauty ("slay that eyeshadow application"), or music ("that beat? slay!"). You might even hear from your supervisors that you "slayed" that powerpoint presentation.