The singer grabbed a meal Saturday in WeHo with the missus, Hailey, and on their way out ... Justin got a little handsy -- but, like, totally solo. Check it out -- the guy was straight up grabbing onto his crotch for several steps as they were cruising by some cars in the lot.

Like we said, this wasn't just a momentary adjustment or scratch -- which we all go through. On the contrary, JB was holding on tight for a little while it seems ... but no word on why.

His pants here do look baggy, so it's possible he was just trying to hold them up -- albeit, grabbing them in this specific area seems a little inefficient ... assuming that's what this is.

Of course, he could also be doing that thing guys sometimes do when they gotta take a leak ... and bad. Might sound gross -- but when nature calls ... pulling a move like this certainly isn't out of the ordinary. Considering how long Justin was holding this pose, it's possible.

Whatever was causing him to touch cling to his manhood ... here's hoping the Biebs got it figured out in the end. As for Hailey -- well, keep on holding him down, we suppose.