The Brief: The slang term "fit check" and hashtag #FitCheck are ways to highlight one's outfit, usually on social media.

DEEP DIVE
“Fit check” usually is a way of saying “check out my outfit.” It’s commonly used on social media paired with a photo of one’s outfit and may be used as a hashtag: #FitCheck.
“Fit” is short for outfit and “check” refers to checking something out, such as with a vibe check or TikTok “check” meme.
fit check pic.twitter.com/41H36wt1d1
— sabrina. (@sabs0ul) February 29, 2020
fit check pic.twitter.com/9BopG7xjTu
— e-ken (@isawken) March 1, 2020