Mat 11:18 For John came neither eating nor drinking, and they say, He has a devil.

"Eating" is esthiô, which means "to eat" and "to consume" but it has some of the same metaphorical sense that "eat" has in English. It means "decay," with the sense of "eating away" at something. It also means "vex" in the sense of "eating at" something. John wasn't a consumer, but he clearly vexed the authorities.

"Devil" is daimonion, which means "belonging to a demon." It is from daimôn, which actually is the noun "demon." The word 'demon" doesn't necessarily mean "evil" (though it seems the way the Jews used it here), but in Greek is used to refer to a controlling spriritual power, inferior to the gods. It was used to mean "knowing" and "skilled" in the sense that we might say, "He is a demon poker player."

The whole idea of ascetism, that is, denying your physical needs, is neither confirmed nor denied by Christ. His sense seems to be that why you do things is much more important than what you do. You can fast for the wrong reasons just like you can eat for the wrong reasons. He makes this clear earlier in Mat. 6:16. Your motives and goals matter more than your actions alone.