Lax means loose or careless.
Lackadaisical means listless or lazy.
There is some overlap here. If you sleep at your desk, for example, you could accurately be described as lax or lackadaisical.
But there is no such word as laxadaisical.
It’s really going to piss me off if all of you cretins spewing this abomination from your cakeholes do so often enough for it to become an accepted alternative.
This error particularly bothers me because lackadaisical is never a necessary word. It can liven when used correctly, but it carries neither denotative uniqueness, nor precision, nor any other characteristic compelling its use.
There are plenty of nifty words that succinctly express something that would otherwise require a phrase. I’ve always loved the word juxtapose for its economy. Onomatopoeia is another good example. Neither word describes anything particularly difficult to understand, but you’re up to five or six words to say the same thing without using them.
Lackadaisical fails that test. If you’re using it, it’s 100% discretionary. You don’t need the word; you want the word. That being the case, you certainly do sound ridiculous when you botch it.