Word(s) that only ever appeared in one (hit) song, ever.

"Tae Bo" is actually an even better pull from that same song (although I could see a rule where "proper nouns" don't count)

Jay-Z bragged about doing Tae-Bo on Girls, Girls, Girls (US #17) and Nas dissed him for it on Ether.

"Proofread" in Zebra Katz's "Ima Read"?

Forgot--"hit song." Well, it was a big hit in my house.

From "Suite: Judy Blue Eyes":

Thrill me to the marrow

"Pitchfork" appears in one hit song.

"Combine Harvester" by The Wurzels.

I thank you.

― Mark G, Monday, 6 June 2011 09:27 (twelve years ago) bookmarkflaglink

Which also has "gaiters", "wurzels" and "gallivanting".

The Wurzels' "I Am a Cider Drinker" has "britches", "tadpoles" and "newts"

("scrumpy" appears in both songs so is disqualified.)

Meanwhile their minor (#32 hit) "Farmer Bill’s Cowman" has "smock", "broody", "faggots" (the foodstuff), "squelching", "drake", "muck", "shovel", "milking", "ploughman" and "Somerset".

isinglass
noun
1. a kind of gelatin obtained from fish, especially sturgeon, and used in making jellies, glue, etc. and for fining real ale.

Blossom Dearie "Surrey With the Fringe On Top".