Question

keep your chin up and i hope you break a leg!It only happens once in a blue moon.what does this idiomatic phrase actually mean?

Answer 1

this is a tricky phrase and has 4 meaning ,which are
1. look upwars and keep smiling .it only happens once every month 
2. stay positive and i wish you good luck it hardly ever happens
3. stop talking and i hope you have lots of fun ! it only happens once a month
4. dont stare at the ground , i wish you would fall over !it happens all the time

Answer 2

keep ur chin up - Cheerful in difficult situation

break a leg - wish a good luck

once in a blue moon - rarely


Hope it helps