a fanny pack is a romantic couple where there is a visible discrepancy between the two parties. in a heterosexual couple it is a fanny pack if the female is attractive and the male is obviously a loser and or looks like an idiot. fanny packs are surprisingly common. far less common is the reverse fanny pack where the male has great merit and the female is visibly a mess. the name comes from the fact that offending males often wear a fanny-pack and also the fact that fanny-packs are embarrassing and "attached at the hip". in the case of homosexual couples where there is a discrepancy treat the more masculine party as the male for determining fanny pack or reverse fanny pack. although simply "fanny pack" is fine in most cases.
oh my god, fanny pack. (said as a qualifying couple passes)
you have to break up. you guys are a fanny pack. (in a serious intervention)