What it means
Being down bad means you're in a rough, desperate state, most often hopelessly into someone and not playing it cool at all. You're liking their three-year-old posts at 2am and sending paragraphs, that's down bad. It can cover any low moment, like being broke or losing badly, but the thirsty, embarrassing version is the one everyone means.
Usage examples
"He's so down bad for her he drove two hours for a coffee she didn't even show up to."
"I'm down bad this month, payday cannot come quick enough."
"Bro was down bad enough to cashapp her just because she posted a sad selfie."
"She's down bad for that man, acting like every vague tweet is a secret message."
Where it comes from
It comes from African American Vernacular English, where down already meant low, rough, or in a bad state. The phrase picked up speed in rap and online talk through the late 2010s, then TikTok helped lock in the now-common sense of being embarrassingly thirsty, obsessed, or desperate over someone.