This song was originally and widely known as "Relax", since that's a prominent word (and theme) of the lyrics. But during 1998, the band started heavy toying with the titles of songs. (For another example, "Oblivious Fool" became "Shafty" and then, according to Tom Marshall, "Olivia's Pool".) What was known as "Relax" was released on Ghost as "Frankie Says", alluding to the early-1980s band Frankie Goes to Hollywood, who had a song called "Relax" which was glorified in shirts reading "Frankie Say Relax" in super-large lettering. Those shirts are now an icon of 80's fashion, and Phish purportedly renamed the song either because there was already a song called "Relax" or ... just cuz they're goofy and want to mock that 80s band.

So when you see Ross on Friends or Homer on The Simpsons wear a "Frankie Say Relax", it is not a Phish reference.

BTW, Frankie Goes to Hollywood returned over a decade later, touring small clubs and holes in the wall from February to August 1999. None of the original members were involved, and the new lead bandmember strained in interviews to explain the new entity's connections to one of the original producers.

"Relax, don't do it, when you want to go through it.
Relax, don't do it, when you want to cum."
-- Frankie Goes to Hollywood