Infix is a noun that refer to letters added to a word to change meaning.

The Latin word figere (fig AIR ay) means ‘to fasten.’ From this word we get infix which has a meaning similar to figere. Although it has other uses, It is generally used when referring to language. Here’s an example:

I got a text from Greg that said he didn’t car about the game. This puzzled me until he later infixed the word ‘car’ to change its meaning to ‘care.’