Sanction

"Sanction" is an interesting word, in that, depending on context, it can have diametrically opposing meanings. Words like this are called contronyms.

A judge may "sanction" a party during a legal proceeding, by which it is meant that he imposes sanctions (penalties).

Conversely, the word may be used to mean "approve of," especially in an official sense. "The law sanctions such behavior" would mean that the behavior spoken of enjoys the specific approval of law.

See also sanctions.