a person who has no real friends their own age and therefore hangs out with their kids friends or their siblings friends (not to be confused with a pedophile, milf, cougar or a douche bag, though loser may be an acceptable synonym)
Steven graduated with me in the 90's but he goes skateboarding with my little brother who graduated last year, he's such a lose bag.
what the kids aren't saying these days
Yo, you're such a lose bag.
WTF is a lose bag?
Not just a loser but whole container of loserness.
Girl1: Matt is an uberloser
Girl2: He's a losebag
A person who loses so hard, they metaphorically morph from a person-shaped bag of meat and blood and stuff, and instead become a big baggy blob full of nothing but lose.
"He flaked out on me again. What a fucking losebag."