1Of a person or animal: that (habitually) violates standards of proper or appropriate behaviour.

2Of an object, phenomenon, etc.: having properties that do not conform to an expected, desired, or predictable pattern; (Mathematics and Physics) having complicated, irregular, or unusual properties that are not amenable to mathematical or numerical analysis; giving unsatisfactory or meaningless results under some particular operation. Contrasted with "well-behaved".

3Computing. Of a program: having an undesirable effect on a computer, either by accident as a result of an error, system compatibility problem, etc., or deliberately (for example in software designed to draw a computer user's attention to advertising by means that the user may find annoying). Contrasted with "well-behaved".