Taking two (or more) times the prescribed quantity of prescription pain killers, usually for the specific purpose of getting high. The phrase refers to the oral ingestion of tablets or capsules only. The most common pain killers used for recreational polymorphing are t3, also called Tylenol #3, with codeine, oxycodone, and hydrocodone.
"Hey, Tommy C! What're you doing tonight?"
"I dunno. What's up?"
"Me and Funk are polymorphing, thanks to my grandma's doctor!"