As you may have noticed, even native English speakers sometimes have trouble with English. And sometimes, when we're trying to think of what to say, we'll end up combining syllables together in a way that might convey meaning. And the strange thing, is that sometimes, it works. (for example, if someone said "umbrellerator" I would know that they meant some kind of machine that has something to do with umbrellas because sticking "-erator" at the end of a word implies something mechanical or automated.) However, sometimes, it doesn't work. And when it doesn't, the speaker ends up with a mash of sounds. And when someone does that the appropriate response is "That's not even a word." That phrase has a hint of derision in it.

It means a word used in the sentence either spoken/written isn't understood by the listener/reader. In English though depending on situation it could because of a few reasons: spelling mistake the context or use of a word that someone doesn't understand. (medical words. very formal or old words or words from fiction) people could use this sentence to add humor if its a sentence they don't understand (as its been used in tv programmes and is a phrase people use a lot with friends to find out meaning to stuff they don't understand). or to be informative if you have a spelling mistake which makes a sentence hard to understand. could also be used just to be rude. The way they say the sentence would help you know which. EDIT: in the examples you listed below your question, the use of swear words and the sentence structure shows its very rude. i would say the person isn't being very helpful though and is really being quite insulting in the sentence examples.

Means that's stupid!😁