it's not worth it; there's no point; the game is not worth the candle

(informal) to take it easy, to have a cushy time

(informal) to learn, to take a lesson

(informal) to lose it, to lose the plot, to lose one's mind

(figuratively) hand in hand, together

(euphemistic) to lose one's life (to die)

the mists of time, time immemorial
/ the mists of time, time immemorial

to the letter

(figuratively, informal) to rack one's brain, to cudgel one's brain, to put one's thinking cap on

(informal, figuratively) to rack one's brain, to cudgel one's brain