폼미쳤다, pronounced as "Pom Mi Cheot Da", is a new slang that young Koreans use online to say that something is impressive. A more familiar word that has a similar definition would be 대박, or "Daebak".
This sentence is a combination of "폼" or "Pom" (form), and "미쳤다" or "Mi Cheot Da" (crazy). It literally means "Your form is crazy", but it has become trendy enough to be used in any situation where you want to say that something is incredible.
Watch this video from Psick University to understand when to use it, and how to pronounce it!