There is probably no single-word term.
This is a rhetorical device, otherwise called figure of speech.
(Wikipedia) A figure of speech or rhetorical figure is a word or phrase that intentionally deviates from ordinary language use in order to produce a rhetorical effect. Figures of speech are traditionally classified into schemes, which vary the ordinary sequence of words, and tropes, where words carry a meaning other than what they ordinarily signify.
Among the second of the two categories, i.e. the tropes, are found two subcategories that this series of usages might concern, the puns and the malapropisms.
(Wikipedia) A pun, also rarely known as paronomasia, is a form of word play that exploits multiple meanings of a term, or of similar-sounding words, for an intended humorous or rhetorical effect. These ambiguities can arise from the intentional use of homophonic, homographic, metonymic, or figurative language.
[…]
A pun differs from a malapropism in that a malapropism is an incorrect variation on a correct expression, while a pun involves expressions with multiple (correct or fairly reasonable) interpretations. Puns may be regarded as in-jokes or idiomatic constructions, especially as their usage and meaning are usually specific to a particular language or its culture.
As the words that are used in order to obtain a rhetorical effect do not exist, the device used is not a pun but a malaproprism.
(Wikipedia) A malapropism (also called a malaprop, acyrologia, or Dogberryism) is the use of an incorrect word in place of a word with a similar sound, either unintentionally or for comedic effect, resulting in a nonsensical, often humorous utterance.
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Humorous malapropisms are the type that attract the most attention and commentary, but bland malapropisms are common in speech and writing.
Note The process of back formation referred to in another answer does not apply since the words do not exist.
Example (Wikipedia)
- "Sure, if I reprehend any thing in this world it is the use of my oracular tongue, and a nice derangement of epitaphs!"
("vernacular"; "oracular" is a word but in this compbination it means nothing, so this is just as if it did not exist. Note that this is a bland malaproprism and not a humourous one.)
As there is no finer analysis of this device in Wikipedia or anywhere else I can get my hand on on the net, there is one thing left to define: a category of malaproprism that are conceived on the idea of the substitution of a word or part of a word that is an opposite. One might then speak of antithetical malaproprisms.
(Cambridge Dictionary) antithetical adjective, formal
exactly the opposite of someone or something or of each other
Someone could comment that ironically the joke is cheesy and dumb itself, but the question is not asking that nor opinion based.
– Sep 01 '23 at 17:07