from Wiktionary, Creative Commons Attribution/Share-Alike License.
superlativeform of wriggly: most wriggly.
Sorry, no etymologies found.
And that's certainly what happened to me, with one of the wriggliest earworms being this little ditty..
I try to do a life saving stroke as I drag him along but he's the wriggliest and bossiest rescue subject ever, and insists that I swim faster and go from left to right.