You know you're blowing up in the music biz whenWeird Al wants to parody one of your songs ... which is exactly what happened Friday to Iggy Azalea -- and we got the whole thing on video.

Weird Al was backstage at KS107.5's Summer Jam in Colorado when he started pleading with Iggy. He's said he always tries to get permission from other artists before doing his thing.

Sources at the show tell us Al was really cool about it -- and even left Iggy a copy of his proposed lyrics. Unclear which Iggy hit he's targeting ... but Al might have dropped a "Fancy" hint with this IG post from the show.

Make sure you watch the clip to the very end ... for Iggy's awesome ending.