Legendary singer Chubby Checker has his panties in a twist over Chubby Checker (the penis-measuring app) ... and now he's suing to the people responsible.

The 71-year-old music legend has filed a lawsuit against Hewlett-Packard and Palm (ha, palm) ... claiming they violated his trademark on the name "Chubby Checker" by releasing "The Chubby Checker" app, which "enables women to estimate the size of a man's penis based on his shoe size."

Chubby (the singer) says he fired off a cease and desist letter back in 2012 ... but the companies ignored his demand, so he was forced to take them to court. According to the suit, Chubby not only wants the companies to shut down the app ... he also wants a ton of money for damaging his reputation.