I said something like "Irregardless of the cost..." and my colleague stopped me.
"There is no such word." He said.
"Just use 'regardless'." He advised.
It made sense immediately. The prefix ir- doesn't add any semantic weight to the word. The tricky part is our natural instinct to add some negation, but regardless already contains the -less suffix. This, of course, creates a double negative.
Merriam-Webster says that the word originated in American speech in the early twentieth century and has never been accepted as correct. The common theory is that irregardless is a confusion of "irrespective" and "regardless".
I certainly learned something new today. Regardless of what I thought, irregardless isn't a word I'll be using any longer.