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 often do irrational or foolish things when they are in love.
I and my boyfriend have decided to get married in the summer. I don't think that's wise, since we won't have enough money saved by then to set up house. But then, one cannot love and be wise.