This is a bit of a scary grammar term (wait! Come back!). It describes single words that have two contradictory meanings. Here are some examples:
bolt: to stick something together, or to run the hell away
dust: to put dust on something, or take it off
peer: someone who’s super-posh, or someone who’s equal
sanction: to approve something, or to boycott it
bound: tied up, or bouncing about
oversight: to watch over something, or to miss or omit it.
(I’ve said it before and I’ll say it again – ENGLISH IS A VERY STUPID LANGUAGE.)
Janus – his name has ‘anus’ in it, tee hee.
The word itself was coined by one Jack Herring in 1962. I’m afraid I don’t know who Jack Herring is, sorry. Although I do enjoy a fish-based name, which is why I’ve included this bit of info here.
The prefix ‘contra’ means ‘against’ or ‘opposite’, which is why ‘contronym’ is also sometimes spelled ‘contranym’ (I think either is fine TBH). Contronyms are also called auto-antonyms, which is boring, and Janus words, which is a bit more interesting. This is after the Roman two-faced god (he literally had two faces – he wasn’t talking behind the other gods’ backs). Janus was the god of (deep breath) beginnings, gates, transitions, time, duality, doorways, passages, frames and endings. He’s got two faces because one is looking towards the past, and the other towards the future apparently (dunno what happens if he turns round). Presumably it’s the two-faced bit which applies to contronyms.