What phrases or sayings, in any language, have you come across that don't seem to make much sense, but are commonly used, anyway?

I am Latin American, and my maternal grandmother lived with us in my childhood home. She would often ask me & my siblings what we were doing or getting into, especially during Summer vacation. We would reply with "nada," which means 'nothing' in Spanish. But, nada also can mean 'he/she swims.' So, whenever we would reply "nada," she would say: "Humph. El que nada no se ahoga." Meaning, "Humph. He who swims, won't drown." Then, she'd simply turn and walk away, leaving us confused as to what in the world she was talking about. Since then, I've heard other Latin Americans use the phrase often, usually older female members of a family, including my aunts and older cousins.