A contranym (or contronym or even autoantonym if you prefer) is a word for which there are two opposite or contradictory meanings.
'Left' can mean remaining or departed. When the girls left the room, the boys were left behind.
'Trim' can either mean adding or removing something. I trim my hat with flowers, but trim the hedge with shears.
'Refrain' is either to stop or repeat depending on the context. I might sing the refrain, or refrain from singing. (If you're nearby, you'll prefer the latter.)
Can you think of more examples?