Used by people who ignorantly mean to say regardless. According to webster, it is a word, but since the prefix "ir" and the suffx (sic) "less" both mean "not or with" they cancel each other out, so what you end up with is regard. When you use this to try to say you don't care about something, you end up saying that you do. Of course everyone knows what you mean to say and only a pompous,rude asshole (sic) will correct you.
A word used by uneducated people intending to sound intelligent. Often, the defendant will use this word in court in an attempt to impress the judge and jury. Educated people notice and those who use this word instantly identify themselves to educated people as being uneducated. Educated people rarely correct them because it helps educated people more easily identify them if they are well groomed.
So, it gives me such joy to copy something from one of the frameworks provided by the state when it says ""Angles are called coterminal if they are in standard position and share the same terminal side irregardless [emphasis mine] of the direction of rotation." [page 5, https://www.georgiastandards.org/Frameworks/GSO Frameworks/Math IV unit 4 SE.pdf]
The students knew it was the wrong word. Why didn't the educators who wrote the book we are supposed to use?