Taylor's rep tells TMZ, "This is a ridiculous claim and nothing more than a money grab. The law is simple and clear. They do not have a case."

Taylor Swift's massive hit "Shake it Off" was not really hers ... so claim 2 accomplished songwriters who wrote a hit song with similar lyrics 16 years ago ... at least that's what they're saying in a new lawsuit.

Sean Hall and Nathan Butler say they wrote a song entitled "Playas Gon' Play" back in 2001, and it Hit #81 on Billboard Hot 100. It was recorded by a big girl group, 3LW.

Among the lyrics ... "Playas, they gonna play and haters, they gonna hate."

Not hard to figure out the rest. Hall and Butler say 20% of "Shake it Off" is their song.

BTW, Butler has worked with the Backstreet Boys, Christina Milian, Aaron Carter and Victoria Beckham. Hall has worked with Justin Bieber, Lionel Richie, Pink and Maroon 5.

Butler is being repped by Gerard Fox Law and Julian Everly Shervington Wright.

They want a ton of money.