You might want to hold onto your undies for this one -- Lil Nas X was spotted completely naked before his arrest this week ... and we've got video of the stunning moment.

Check out our clip -- he's strolling down the street while singing after ditching his underwear and boots somewhere in Los Angeles. He seems completely unfazed by his lack of cover ... even making eye contact with the camera as he puts on a show.

And if you didn't notice ... he's belting some Nicki Minaj lyrics, specifically her line in "Monster" by Kanye West .

We're told he was spotted running free around 5:45 AM Thursday, not long before police picked him up and had him transported to a hospital for a possible overdose.

TMZ learned bystanders had reported to LAPD that they saw a nude man walking d

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