Everyone and their mother phrase informal
A large number of people
When I went to the cinema last night, everyone and their mother was there.
Everyone and their mother will come to my birthday party tomorrow.
A group of bad or sinful people
To count the number of people in a group to determine who is present
If you fall for someone's trick the first time then they are at fault.
Are you going to mess with me again? Fool me once, shame on you; fool me twice, shame on me.