The singer is still out in Italy, enjoying a little R&R after completing his final show in Reggio Emilia. On Friday, he went on a little boat trip on a lake in Bolsena -- where he and some pals (James Corden included) were soaking up the sun. And, yes ... HS went shirtless.

Besides the fact that he's got a ripped bod, he's also got a ton of tattoos all over his torso. More importantly, though, he has leg tats as well -- one in particular here stands out.

Peep ... it's ink on his thigh, of all places, and it clearly reads "Olivia." There's another word right underneath that, but it's harder to make out what that one is, or what the connection is. Of course, for the Olivia tat ... folks are obviously assuming it's related to his recent ex.

There's another possible explanation ... in 2015, One Direction put out a song called 'Olivia,' so this could be related to that. Considering we haven't seen this tat on him before, though -- many are speculating it's fairly recent. If it is ... well, that's certainly interesting.

As you know ... Harry and Olivia dated for the better part of a year or so, and they seemed head over heels for quite a long time. Of course, their relationship wasn't without drama.

Seeing how Harry dating Olivia caused a lot of strife (at least among his fans) ... ya gotta wonder if he'll have this removed at some point -- again, assuming it is in fact related to her.