The lyrics of Bobby "Boris" Pickett's classic 1962 Halloween jam "Monster Mash" mostly make sense, with one weird exception: The background vocalists sing about a tennis shoe. Why? Nobody knows and even Pickett himself couldn't remember during a Dr. Demento interview in 2006, the year before he died.
"[They're saying] 'Ooh, tennis shoe, wah-ooh," he told Dr. Demento. "We don't know why."
From The Hill:
[The song] was written by Pickett and Lenny Capizzi, his bandmate in a group called The Cordials. The idea came about after Pickett began demonstrating his Boris Karloff impressions during the band's cover of "Little Darlin'," a 1957 song made famous by The Diamonds. Specifically, Pickett would perform the song's monologue ("My darlin', I need you…") in his Karloff voice, much to the audience's amusement.
(Thanks, Bob Pescovitz!)