This is a repeat, but I like it enough to run it again. Contrast with a prefix, an affix added to the front, and suffix, added to the end, of a word. This is not done much in standard English, but there are slang uses such as shiznit for shit or edumacation for education (both of which I think are perfectly cromulent, but I'm not a linguist). The insertion of a whole unfuckingbelievable word is technically a form of tmesis. From Latin, which does have plenty of infixes, and not coined in English by grammarians as I expected.
You can comment here or there.