Antonio Brown just issued a massive apology for his off-the-field behavior -- but there's one huge problem with his mea culpa ... it's unclear what he's specifically apologizing for.

"First and foremost, I’d like to apologize to my family, friends and anyone who I offended," Brown said on IG.

"I never once meant to make anyone feel like I wasn’t thankful and appreciative of the opportunity that I was afforded to play the game I love."

Here's where things get even murkier ... AB references "inexcusable" behavior -- but doesn't specify if he's talking about serious allegations like rape or stupid incidents like farting on his trainer.

"While my behavior was inexcusable, sometimes when people are coming at you with false information and allegations, we handle ourselves in ways that we sometimes regret."