'Always Sunny' Star Charlie Day -- I've Tried Rum Ham ... IT'S DISGUSTING!

"It's Always Sunny in Philadelphia" star Charlie Day has ACTUALLY taken a sip of rum ham ... and even though his crew made the drink famous, he says it's vile.

If you haven't seen the show -- first of all, shame on you -- rum ham surfaced in Season 7 when Frank (Danny DeVito) concocted the rum-soaked delicacy.

Day was retrieving his bags at LAX on Saturday when he told us he was once at dinner ... and someone sent a rum ham drink to his table -- but you gotta hear Charlie tell the story himself.

Still beats sunscreen tequila.