You can’t have your cake and eat it too means that you can’t have it all i.e. you can’t have everything you want, especially two desirable, but conflicting options!
Got money to burn stems from the times when poor people could only afford simple wax candles to light their homes. The rich however, could afford expensive beeswax candles which were of higher quality and lasted longer. Therefore, they had ‘money to burn’. Today the expression is used when talking about someone who can afford luxuries that others can’t.
It’s raining cats and dogs simply means that it is raining very heavily!
Once in a blue moon is an expression used to describe an event that happens rarely, if ever. When did you last see a blue moon?
And pigs might fly… Very similar to ‘once in a blue moon’. You might hear it used like this: Sam said he’d pay me back tomorrow… and pigs might fly. Sam obviously doesn’t have a reputation for paying back on time!
Look at Sally and Jane… they’re getting along like a house on fire! In other words, they get along really well, with no difficulty at all.