Contronyms...

A synonym is a word that means the same as another. An antonym is a word that means the opposite of another. A contronym is a word that, by some freak of language evolution, are their own antonyms.

Here are a few examples of contronyms:

bolt - secure, run away
cleave - separate, adhere
custom - usual, special
fix - restore, castrate
grade - incline, level
left - remaining, departed from
fast - quick, unmoving
screen - show, hide
weather - withstand, wear away
wind up - end, start up (e.g., a watch)