It's been nearly five months since the Chiefs lost Super Bowl LV ... but Tyreek Hill is still FUMING over the L -- telling TMZ Sports the defeat was "embarrassing."

Hill spoke candidly this week about February's 31-9 loss to Tom Brady's Bucs ... admitting it's fueled him to work out like a mad man this offseason to ensure that NEVER happens again.

"Man, you know what, man?" Hill said. "I've been grinding my tail off ever since that loss."