If something has fallen off, it might mean one of several things. It could mean that an object has slipped off the surface it was sitting on, and fallen. It could also mean that something has decreased.
Examples of both meanings:
"Write it off" means to dismiss something.
To laugh allot
The meaning of the idiom "to slap the back off you" is fairly straightforward. It implies an exaggeration, that one would slap someone else so hard that their back would come off.
Something that has fallen through can mean several things. One thing is that the thing has slipped through a crack or opening and fallen. Another meaning is that something which was supposed to occur has not.Some examples of each meaning:I dropped my money and it has fallen through the grate in the sidewalk.The merger has fallen through, and we need to find a new business partner.
Can you figure out the meaning by defining the terms literally? No, so it is an idiom. Literally, it means to remove something, but figuratively it means for an airplane to get off the ground.
Showing off, being well dressed and flashy.
To lose ones temper and react violently.
Old sailing slang meaning to be no longer in danger.
It means being ticked off, or very mad.
Nothing. To get off the hook, however, means to escape punishment.
"To be" is not an idiom - it's a verb.
That usually pertains to a weapon discharging when you don't want it to.