Tmesis is a linguistic device in which a word or phrase interrupts another word or phrase. (The word is a Greek term that refers to cutting.) Depending on the type of tmesis, it is either acceptable in formal usage or relegated to humorous and/or emphatic colloquialisms.
Phrasing in which the preposition down is located within the verb phrase “turn down” in “Turn down that music,” as opposed to its placement in “Turn that music down,” is a standard form of tmesis, as are whatsoever and unbeknownst, in which, respectively, so is inserted in whatever and be is placed within an archaic form of unknown. (Interestingly, in some literary usage, a tmetic word is itself cloven, as in the biblical verse “He shall be punished, what man soever offendeth.”) By contrast, seemingly tmetic words such as notwithstanding and nevertheless do not qualify, because the framing syllables do not constitute words or set phrases.
A form of tmesis often heard spoken spontaneously but best reconstructed for writing is a possessive phrase such as “the girl in the back row’s,” referring to something belonging to a girl sitting in a back row; the modifying phrase “in the back row” is artificially inserted between girl and the possessive s. “The book is the girl in the back row’s,” for example, should be recast as “The book belongs to the girl in the back row.”
Informal tmetic usage is ubiquitous but discouraged in formal writing. Examples include “a whole nother” and “any old how” as intensifications of another and anyhow. Recently, however, this form of tmesis has been supplanted in popularity by a form formally known as expletive infixation, in which a profane or otherwise emphatic word is inserted into an adjective to fortify its impact, as in abso-frickin’-lutely and la-dee-frickin’-da. Another colloquial construction is the emphatic insertion of so in such statements as “I am so not going there.”
These contemporary conversational habits have their place in transcriptions of casual dialogue and in light-hearted informal prose, but they’re intrusive in formal writing.