irregardless: regardless
—Merriam-Webster Dictionary

Etymology
Irregardless is likely a blend of irrespective and regardless. People have been using irregardless since at least the 1870s.[i]

Usage controversy
An easy way to elicit groans from your snooty peers is to pepper your conversation with the word irregardless. “That’s not a word!” you’ll hear them cry. However, take a look at this usage note in the Merriam-Webster Dictionary:

The most frequently repeated remark about it is that “ there is no such word.” There is such a word, however. It is still used primarily in speech, although it can be found from time to time in edited prose. Its reputation has not risen over the years, and it is still a long way from general acceptance.[ii]

Should you use irregardless?
Irregardless of people’s general hatred of this word, should you use it? Technically . . . no. The prefix ir- means not. So when you say irregardless, you are actually saying not regardless. This is likely to be the opposite of what you intend, since the definition of irregardless is regardless. To avoid confusion, and the caterwauling you are likely to encounter in the seconds after you say the word, it is best to stay away from irregardless.

But—I must admit that I, as a professional copy editor and official word nerd, love irregardless. Using irregardless is a cheap way to get laughs from fellow editors. However, I never use it around people I’m not familiar with and who don’t know my borderline obsession with this word, lest I be branded with a scarlet letter I in social circles.