If people behave in such a way that you find it almost impossible to accept that they actually did it, then you can say that their behaviour was beyond belief.
If you bite off more than you can chew, you take on more responsibilities than you can manage. 'Don't bite off more than you can chew' is often used to advise people against agreeing to more than they can handle.
People say '(well,) blow me down' when you have just told them something surprising, shocking or unexpected. ('Blow me down with a feather' is also used.)
(NZ) Used when something is working well or going your way. If you are happy, you are a box of fluffy ducks. Also can be shortened to 'a box of fluffies'.
If people are boxing and coxing, they are sharing responsibilities so that one of them is working while the other isn't. It can also be used when couples are sharing a house, but their relationship has broken down and when one is at home, the other stays out.
If you break ground, or break new ground, you make progress, taking things into a new area or going further than anyone has gone before.
'Ground-breaking' is used an adjective.