folie à deux

• folie a deux (without diacritics)
• folly à deux, folly a deux (rare)

Borrowed from French folie à deux (“madness for/of two [people]”).

• IPA^(key): /ˈfɒli ə ˈdɜː/

folie à deux (plural folies à deux)

1. A situation in which two people behave abnormally together in a way that neither of them would on their own.
2. (psychiatry, strictly) A rare syndrome in which a symptom of psychosis (particularly a paranoid or delusional belief) is transmitted from one person to another.

• à deux
• ménage à trois