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
People tend to like forbidden or illegal things just because they are forbidden or illegal.
He must have believed that stolen fruit is the sweetest, and he, therefore, cheated on his partner.