You found an interesting phrase. It is not about 身＝body nor 蓋（ふた）=lid of pots. It means " You say things too straightforward, without euphemism. " So here's 2 example conversations... ex.1 A: きのうジムの会員になったんだ。I joined a gym yesterday. マッチョになって、モテモテさ！( I will become a big muscle man and all girls will crash on me!) B:お金のむだだよ。it's waste of your money. ジムの新会員の半分以上がひと月以内にやめるって知ってる？ Do you know more than half of new members quit gym within a month? A:　そんな身も蓋もないこと言うなよ。(Hey, don't say that. It's brutally straightforward!) --- ex.2 A:なんで、あんな　いやなやつと　つきあってるの？ Why are you dating with such a jerk? B:だって、彼って　ちょうお金持ちなの。Because, he is so rich! A:身も蓋もないなあ。(Oh no... you are too straightforward.) Hope it helps. (-;