A contronym is a single word with two opposite meanings, since to dust can mean to add or remove dust.

The longer version

Dust the cake means sprinkle sugar onto it, while dust the shelves means wipe particles off; the verb simply adds or subtracts depending on what the object already has. English forms verbs from nouns very freely, and the prefix or particle that would separate the two senses in another language is left unstated.

Cleave behaves the same way, meaning both to split and to cling, and sanction covers both approving and penalising. Such words usually began as two separate verbs that happened to converge in form, rather than as one word that reversed itself. Context settles the reading instantly for native speakers, which is why the clash goes unnoticed until someone points it out.

Where this entry sits

This category gathers small surprises about the English language, from odd spelling rules to words that quietly hide their own history. Expect palindromes, punctuation oddities, borrowed terms, and the strange ways writing systems record spoken speech. The full list sits on the Words and Language page, where every entry is listed in order.