To take a rain check (on something) informal idiom.
Meaning: to decline an invitation/suggestion meaning to do it later (not saying when exactly).
Origin: in 1880's in the USA some activities were cancelled due to heavy rains. In this case in baseball, for example, the ticket holders were given 'rain checks' special vouchers that gave the right to visit the next game free of charge.
Such practice was also held in grocery shops. Say, you went to the shop to buy milk but the milk is sold out and new supply wasn't delivered because of the rainfalls. You take a voucher with current price for milk and can buy the milk later for this price even if the price goes up.

Prepared by Viktoriia Burlachenko