‘Metathesis’ is a linguistic term (wait, come back – it’s interesting, honest!) which basically means to swap bits of a word round to create a new one. The word ‘walrus’ came about because of metathesis. It’s from an old Norse word ‘hrossvalr’ which means ‘horse whale’. At some point when the word made its way over to us, somebody switched it round (it was possibly more complicated than this) and we got ‘walrus’. ‘Foliage’ is another example. The word comes from a Latin root (BOOM BOOM), ‘follium’ which means leaf. Metathesis happened to it at some point and it went from ‘foillage’ to the better known ‘foliage’ we use today.
The most famous modern (or is it…?) example of metathesis is ‘aks’ for ‘ask’. ‘Aks’ actually came first from the Old English word ‘acsian’. Because of metathesis in ye olde times (scientific, I know), there was also another version floating about – ‘ascian’ – which won the linguistic fight and is how we ended up with ‘ask’ being the norm. (I used to absolutely loath it when I heard people saying ‘aks’ instead of ‘ask’, but now I know it’s from Old English and that Chaucer used it, I don’t feel so cross about it. Because I’m a pretentious wanker apparently.)
The word metathesis itself comes from the Greek word ‘metatithenai’, which just means ‘to put in a different order’. So that’s not very interesting, sorry. There’s also a super-poncy joke in here about metathesis being a thesis about a thesis, but I’ll spare you. Because I don’t think it’d be very funny, even to me.