Mighty wide

Somewhere along the line I heard a phrase, "that's mighty wide of you." Now, I'm probably just naive, but I only had a vague notion that this was considered crass. Then I heard someone mention that they were quite offended when they heard a white person use the phrase...the offended party was black, by the way. Or, uhh...should I say African American? Anyway, is there some deeper, more sinister meaning to this phrase, or is it cultural slang that "shouldn't" be tossed around by those of another culture? Thanks!