身も蓋もない is a vivid Japanese idiom meaning that something is so direct and unfiltered that it leaves no room for tact, euphemism, or imagination. Literally, it suggests having neither the contents nor a lid, which gives the phrase its memorable image.
In the song, the narrator uses it while rejecting gloomy or sentimental talk, adding a sharp, almost defiant tone to the lyrics. It is a distinctive expression that appears repeatedly and offers learners a fascinating glimpse into Japanese idiomatic language.
"Bedtown" is a song about two people stuck somewhere dull who decide that being stuck together is enough. A bed town (ベッドタウン) is the Japanese word for a commuter suburb, the kind of place people only sleep in before taking the train back into the city. King Gnu open with an escape plan: 永遠の雨の中から飛び立つんだ (we're taking off out of this endless rain), and later 退屈なこの街角から抜け出すんだ (we're breaking out of this boring street corner).
The singer is surprisingly honest about his limits. 昔なりたかった自分には多分なれやしないだろう (I probably won't ever become who I wanted to be) is a blunt thing to admit in a chorus-bound rock song, and he takes daily "blows to the solar plexus" (鳩尾, mizuochi, the pit of the stomach). Watch for the idiom 身も蓋もない, literally "no body and no lid", which means too blunt to be any fun. He keeps shrugging it off: けど湿った話は嫌いだよ (but I hate soggy, gloomy talk).
The answer he lands on is small and warm. このまま二人で 僕らなりの歩幅でいいと思った (I thought it was fine, just the two of us, at our own pace). Even being 群れからはぐれた (strayed from the herd) feels precious, and そうさ それが全てだろう (yeah, that's everything, right?) sums up the whole mood. Learners get some great rough, casual speech here too: 知るかよ (like I care) and 勝手にしやがれ (do whatever the hell you want).