Okay, this isn't responsive to your query, ASp, but i thought i'd share:

i spoke to a friend of mine who was born and raised in Rio, and she didn't recognize the expression at all, but a couple of hours later she called back, saying the curiousity had gotten the better of her so she'd called her sister (who evidently still lives there). anyhow, her sister recognized it immediately, and said it's a "colloquial" term (wow, your portuguese must be *awesome, Anna!), and she added this bit of info:

"manca" means (as she put it) "people who don't have a clue". the example she used was a person who arrives uninvited during the dinner hour and has absolutely no idea that their presence is intrusive. i asked for a usage example and she offered this: "Ele neo se manca", which supposedly means "he's a clueless git". (or am *i the clueless git, and she's tricked me into writing "i'm a clueless git" in portuguese, since she knows i don't speak a word of it? )