"A kick in the teeth" is an idiomatic English expression used to mean an unpleasant surprise or setback. It often has the more specific sense of a bad outcome which occurs instead of an expected good outcome. The saying relies on a simple metaphor, implying that an event is as painful, discouraging and humiliating as being kicked in the mouth.
"Kick in the teeth" is one of a large number of idiomatic English expressions, dating back to the 18th century, which relate to being kicked. The song "Ain't That A Kick in the Head," made famous by Dean Martin, uses "kick in the head" to refer to a sudden shock, while "kick in the pants" has a similar meeting. Similarly, to defeat an opponent is "to kick his butt," while to summarily eject someone from a business is "kick her out." Interestingly, there is no similar range of expressions relating to punching. In English slang, being kicked seems to be somehow more humiliating than being punched, perhaps because of the visual and physical dominance suggested by the gesture of kicking.
The experience described by the expression is more shocking and humiliating than some similar expressions. "A kick in the head" and "a kick in the pants" can both be the necessary shock that forces a person to change perspective. "A kick in the teeth," however, is never used in this sense -- the experience is never salutary, but always painful and frustrating. The difference may be that blows to the head and the seat of the pants are frequently used for comic effect in the media, while being kicked in the teeth is a more violent and frightening image.
Another use of the expression is comparative, as a part of the longer idiom "better than a kick in the teeth." This is used to indicate grudging acceptance, acknowledging that while something is unsatisfactory, it could be worse. It is often used ironically, as a deliberately understated response to something of which the speaker actually approves. For instance, someone receiving an unexpected windfall might say "a million dollars? Well, I guess that's better than a kick in the teeth."
The idiom is widely understood, and is used in both American and British English. As such, it appears in a variety of media. For example, a 2010 single by new-metal band Papa Roach is entitled "Kick in the Teeth." The same phrase occurs as the title of singles by Supergrass and Fischerspooner.