You probably know that synonyms are words that have similar meanings and antonyms are words that have opposite meanings. But a contronym is a single word that can have two opposite meanings.
For example the word "sanction" can mean either to show support of something (give permission, allow, bless) or to not support something (punish, to penalize). Sanction is a contronym. You talk about a marriage being sanctioned, as well as countries being sanctioned, and it means opposite things contextually.
Here's some other example's of contronyms and some sentences that use the word both ways:
- Cleave (Adhere, Detach): "She loves to cleave tree trunks with her father's old axe, when I tried to buy her a new one she cleaved to it and would not let the old one go."
- Clip (Adhere, Detach): "After she got her hair clipped she clipped in a hair bow"
- Dust (Remove particles, lay down particles): "He dusted off the crumbs before he dusted them with sugar."
- Left (Departing, Remaining): "After the girls left the room only the boys were left in the room."
- Variety (One type, many types): "There was a variety of pears at the farmer's market, but I only bought Red d'Anjou pears because they are my favorite variety."
- Weather (Withstand, wear away): "After years of weathering big storms, the barn became weathered after last year's hot summer."