abuela, noun - grandmother
Family ties tend to be strong in Spanish-speaking countries, so la abuela, grandmother, often has a key role. And that’s probably why there are various idioms using the word. Be careful: these are rather informal, so it’s probably best not to use them, but just to know what they mean.
If person A is telling person B something that B just doesn’t believe, B might say:
¡Cuéntaselo a tu abuela! (Literally, "Tell it to the (your) grandmother.")
Tell it to the Marines, or (British) Pull the other one; it’s got bells on.
And when something goes wrong after lots of other things have gone wrong, you might hear:
Como éramos pocos, y parió la abuela. (Literally, "When we were little, and Grandmother stopped.)
That was all we needed, or As if we didn’t have enough problems.