Conor McGregor went balls-out during his most recent training session ... putting his bulge on display as he sweated it out in a pair of Versace undies -- and it looks like a scene straight out of "American Psycho."
The Notorious posted the odd video on Tuesday ... showing him crushing it on a stationary bike as he jammed to Robert Palmer's "Simply Irresistible."
If the clip makes you think of Christian Bale's Patrick Bateman character from the 2000 flick, it's by design -- Conor captioned the social media post, "American Psycho II.