mettre quelqu'un en quarantaine

To exclude someone from a group

Beyond its medical meaning of isolating a sick person, the expression is used figuratively to describe cutting someone off socially, freezing them out of a group or community.

Depuis qu'il a trahi son équipe, ses coéquipiers l'ont mis en quarantaine.

Ever since he betrayed his team, his teammates have put him in quarantine.

The word 'quarantaine' has been recorded since the 12th century and comes from 'quarante' (forty). Mediterranean ports formalised a 40-day isolation period for ships suspected of carrying plague from the 17th century onwards. The number forty already carried weight in Christian tradition (Lent, Christ's fasting in the desert), which likely helped it stick.