IPA: /ˈɪəˌwɜːm/
KK: /ˈɪrˌwɜrm/
A catchy song or tune that keeps repeating in your mind, often without your control.
That song is such an earworm; I can't stop humming it!
Earworm is a compound word formed from "ear" (from Old English "eare", meaning the organ of hearing) and "worm" (from Old English "wyrm", meaning a small creature or serpent). The term describes a catchy piece of music that gets stuck in one's head, much like a small creature that burrows in.
Think of an 'ear' as the organ that hears and a 'worm' as something that burrows in — an earworm is a tune that burrows into your mind.