Tmesis is a linguistic phenomenon in which a word or phrase is inserted into the middle of another, often coming between the parts of a compound word or splitting an infinitive: ‘to boldly go’ or even ‘Jesus H. Christ’. Tmeses are often employed for humorous effect, though a great many are unconscious uses of language in natural ways.
A subset of tmesis, dystmesis occurs when the insertion is (somewhat surprisingly, perhaps) between syllable boundaries within a word: ‘a whole nother’ (’whole’ into ‘another’). ‘A whole nother’ may seem strange to see written out, but if spoken in conversation would likely pass completely unnoticed.
Expletive infixation is a dystmesis where the inserted word is a profanity: ‘abso-fuckin-lutely’ or ‘guaran-god-damn-tee’.
In a similar way, the (meaningless?) particles -iz- or -izn- are inserted into words in hip-hop-slang: ‘hizouse’ (for ‘house’) or ’shiznit’ (for ’shit’), although these aren’t technically tmeses since -iz- and -izn- aren’t words in themselves.
Apparently tmeses are common in Australian speech, where they’re known as tumbarumba.