Shiggy -- the dude behind the "In My Feelings" challenge -- had a real-life dance battle with Backpack Kid at an ESPYs after-party on Wednesday ... and went BADLY for the BPK dude.

First off, we love Backpack Kid -- he's always been super cool to us -- but, man, SHIGGY JUST CAUGHT A BODY!!!

At one point, Shiggy DANCE-ATE Backpack Kid's heart.

The athletes were lovin' it ... everyone from Lakers star Josh Hart to Rams RB Todd Gurley to new Magic center Mo Bamba to Panthers wideout Devin Funchess had their cameras out, egging the whole thing on.

R.I.P. Backpack Kid.