Soccer superstar Lionel Messi just revealed the secret he's been keeping underneath his shinguard -- a brand new piece of body art ... and it's not going over very well in Europe.
It's a pretty simple design ... soccer ball, jersey number, sword, some roses -- but they're killing him for it overseas.
Some major media outlets are calling "terrible," "awful" and "the worst" ... and the Barcelona star isn't getting treated any better on social media:
-- "F**k your tattoo messi why you destroy your leg with this ugly s**t tattoo why please tell him to remove."-- "That's an ugly tatoo..."-- "All that talent can't account for taste"