Contronyms

A contronym is one of the strangest vocabulary issues in the English language. One word can have two meanings, that’s common, but with a contronym the two meanings are opposite! (“Contra” means opposite, such as in “contradict” and “contravene”)

The most common example is “fast”:
The printer moves very fast. (moves with high speed)
The printer head is stuck fast. (moves not at all)

Here are some more:
“Custom” – this can be a common practice (a family dinner is a Christmas custom in the UK) or it can mean a special case (my guitar is a custom model, made to my specifications)
“Clip” can mean to attach (like a paper clip) or detach (such as to clip an article from a newspaper, or clip the leaves off a plant)
“Bolt” can mean to go speedily (the horse bolted from the enclosure) or to fasten (be sure to bolt down the cover or the wind will blow it away)
“Dust” can mean to add fine particles (as in crop dusting, or dusting a cake with sugar) or to remove them (I’ll dust the room)
“Go” means to proceed (It’s going well) or to weaken/fail (Oh! My back has gone!)
“Left” can mean to go away (He has already left) or to remain (There’s some food left)
“Off” can mean to start (the alarm went off) or to stop (turn the alarm off)
“Oversight” can mean to monitor and watch, or to miss something (Sorry, the error was an oversight of mine)
“Refrain” can mean to stop doing something (please refrain from talking) or to repeat (after this verse, sing the refrain)
“Sanction” can mean to approve or to boycott
“A variety” can mean one type (I like this variety of cat) or many kinds (Let’s get a variety of doughnuts)

There are many more, so feel free to add any of your own ideas!