A sneer is a facial expression of scorn or disgust characterized by a slight raising of one corner of the upper lip, known also as curling the lip or turning up the nose. In The Expression of Emotions in Man and Animals, Charles Darwin defined a "sneer" as "the upper lip being retracted in such a manner that the canine tooth on one side of the face alone is shown" Darwin related the sneer to the snarl observed in non-human animals, particularly carnivores, observing that:

The uncovering of the canine tooth is the result of a double movement. The angle or corner of the mouth is drawn a little backwards, and at the same time a muscle which runs parallel to and near the nose draws up the outer part of the upper lip, and exposes the canine on this side of the face. The contraction of this muscle makes a distinct furrow on the cheek, and produces strong wrinkles under the eye, especially at its inner corner. The action is the same as that of a snarling dog; and a dog when pretending to fight often draws up the lip on one side alone, namely that facing his antagonist.