Words that can have virtually opposite meanings depending on context.

As an example, the original/base meaning of to moot is to discuss or debate, as in moot court. Thus a moot point would be something to be discussed or debated, with arguments for and against, yet to be agreed.

As a contranym, a moot point is one that is worthless, uninteresting, of no practical value, indeed not even worthy of debate, best ignored.

Here are a few more.

  • Certain: a certain person vs for certain.
  • Left: those who are left vs those who have left.
  • Original: the original spelling of a word vs a highly original spelling of a word
  • Out: the lights are out vs the stars are out
  • Below par can mean good or bad depending entirely on whether you’re talking about golf or not.

There are heaps of them.