everyone and their mother

Pronoun

idiomatic

(idiomatic) A large number of people; most people.

Japanese Meaning

大勢の人々 / ほとんどの人々 / 非常に多くの人々

When the new cafe opened, everyone and their mother showed up for the free pastries.

新しいカフェがオープンしたとき、大勢の人が無料のペストリーのために押し寄せた。

Sense（1）

(idiomatic) A large number of people; most people.

2011, "Now Everyone And Their Mother Is Shorting China, Especially These Four Stocks", Business Insider