Something that you say when you cannot accept someone's invitation to do something but you would like to do it another time. A - "Shall we meet for lunch?" B - "I have some errands to do, can I take a rain check?" A - "No problem, maybe tomorrow."

-a ticket stub entitling the holder to admission to a future event if the scheduled event was cancelled due to rain -a promise that an unaccepted offer will be renewed in the future -Is the coupon given to the player (under the golf course stipulated conditions and authorized by the management) when it rains in the middle of a ...

rain check –noun 1. a ticket for future use given to spectators at an outdoor event, as a baseball game or concert, that has been postponed or interrupted by rain. 2. an offered or requested postponement of an invitation until a more convenient, usually unspecified time: Since you can't join us for dinner, we'll give you a rain check. 3. a ticket, coupon, or the like, entitling a customer to purchase at a later date and for the same amount a sale item that is temporarily out of stock. http://dictionary.reference.com/browse/rain%20check

What's the context? Maybe it is something you can do AGAINST the rain. To keep it off (/away). Like an umbrella, a raincoat, or even a roof.