In the game of Go (Wei Qi/Badouk), the term "tiger's mouth" refers to the following arrangement of stones:
(no stones at a, b, c, d or e, but see below)
It is generally a good shape, but better in some contexts than others. It is strong, in that it can't be cut, and flexible, in that any of the three stones can be sacrificed without losing the other two. However, it is not always a great idea to form this shape as part of a wall, since if the opponent has stones at b and c, he can play atari at d, and thereby cause you to end in gote (assuming b and c are strong enough that he doesn't need to defend them in turn).
Also note that another stone of the same color at a would form a ponnuki, while one at either e would be a trumpet connection. Both of these are wonderful shapes, so the tiger's mouth is often formed so as to threaten making one or the other.