Frequently described as “words that are their own opposites,” Janus words are also known as contronyms, antagonyms, or auto-antonyms. These are words that have developed contradictory meanings. Cleave is often cited as the go-to contronym: it can refer to splitting something apart and uniting two things. But it’s not the only one out there, and there is usually some sort of logic behind most auto-antonyms.
As well as cleave: clip, fast, oversight, sanction. All have contradictory meanings which arise from a word’s original specific definition becoming much broader. Or, somewhat appropriately, the opposite.