[vacri]

No, everyone doesn't 'know what it means', not without additional mental parsing. Myself, among many others, read it first as a double-negative, then have to double-take it and reinterpret it. Irrespective, irreducable, irrecoverable, irreconcilable, irrelevant, irreligious, irrefutable - irregardless stands out like a sore thumb.

Or should I say 'irreinterpret it'? After all, you would 'know what I meant' if I wrote that.

Ambiguity is fine if you're writing for poetic effect, but if you're trying to get a point across, it should be minimised.

[DanBC]

Wait, are you really sYing that when you see "irregardless" that your first response is not "that's an error and they mean regardless[1]" but "they mean regardless but negated so regard but that doesn't make sense so they actually mean regardless"?

[1] why not irregard instead of regardless?

[vacri]

"that's an error and they mean regardless" = "additional mental parsing"

[bane]

Yet astonishingly you are one of the people who knew what it meant.

Also, most English dialects don't strictly follow double-negation rules. I know this drives non-native speakers crazy, but you'll get over it with enough practice.

[vacri]

I think you missed my point - perhaps you should irreinterpret my comment.

[thizzbuzz]

It stands out because "regardless" and "irregardless" are adverbs, while the others you listed are all adjectives.

And it does fit the pattern of negating the root morpheme.