כָּסַל

From LSJ

τίς δ' οἶδεν εἰ τὸ ζῆν μέν ἐστι κατθανεῖν, τὸ κατθανεῖν δὲ ζῆν κάτω νομίζεται → who knows if life is death, and if in the underworld death is considered life

Source

English (Strong)

a primitive root; properly, to be fat, i.e. (figuratively) silly: be foolish.