Or a word to describe the act of inaccurately using complicated or unusual words (often in an attempt to sound more intelligent)?
I considered 'bombastic' but it doesn't have that quality of inaccuracy.
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The act of using a word inaccurately could be called catachresis. Catachresis is defined as: "the misuse or strained use of words, as in a mixed metaphor, occurring either in error or for rhetorical effect." Or as: "The use of a word in a way that is not correct, for example, the use of mitigate for militate.
It has the adjective form of catachrestic.
This entry in Wikipedia says catachresis can be either unintentional or intentional. It's a fun word to say. Just be sure YOU use it correctly.
I think your description of this person requires two words:
1 "inaccurately using complicated or unusual words" = ignorant.
2 "in an attempt to sound more intelligent" = pompous, pretentious
Although bombastic, pompous, and pretentious are often synonymous, each has a different connotation and use. Of these three, I think that only pretentious implies the probability or possibility of ignorance and misuse of words. Pomposity and bombast imply overdramatization rather than deceit.
The word you're looking for is acyrologia. The person who uses such words could probably be called an acyrolog, although that's a bit of a neologism.
If the words being confused are similar sounding, you're dealing with a subcategory of acyrologia called a malapropism or (less frequently) a dogberryism. Mrs. Malaprop is a character in The Rivals by Richard Brinsley Sheridan. Dogberry is a character in Shakespeare's Much Ado About Nothing.
Catachresis can also be the misuse of words, although it connotes an intentional misuse done for a rhetorical purpose.
Given the description, and if you're trying to use it to give something a bit of colour rather than looking for the technical word to describe the way the words are used, I think my favourite descriptive pick would be bluster, or blustering. While not a definite hit for the specific case of using long or complicated words, it certainly carries a self-important air along with the implication of inaccuracy or exaggeration, and the specifics could easily (and, perhaps, entertainingly) be elaborated on later.
I think you’re actually looking for two different concepts.
Where trying to sound more intelligent is concerned I’d be tempted to use the word affected, in the sense of “assumed or displayed artificially; put on for effect; artificial, stilted, ‘got up’ ” (source Oxford English Dictionary).
There was a great line in an episode of Frasier where, describing such a person, Frasier said, “Nothing is quite so irksome as affected erudition.”
In terms of using words incorrectly, incorrect or any of its variants will do.
The word you're looking for is malapropism.
The term comes from the 18th Century play The Rivals, which satirises the tendency you have described. In the play there is a character called Mrs. Malaprop who habitually confuses impressive-sounding Latinate words to great comic effect. Her name reflects the phrase mal a propos, which is borrowed from French and means ill-suited.
If I understand you correctly, you're mainly referring to the scenario where someone simply uses big incorrect words simply to sound intelligent or smart.
I personally don't think there's currently any one word for this but this can often be observed in people who have low self-esteem and often want to exaggerate their importance.
In that case, I think you can call them grandiose and they can be said to be ego-deprived.