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οὖσα

Γελᾷ δ' ὁ μωρός, κἄν τι μὴ γέλοιον ᾖ -> The fool laughs even when there's nothing to laugh at
Menander

French (Bailly abrégé)

ης;
fém. de ὤν, part. prés. de εἰμί.

Russian (Dvoretsky)

οὖσα: f к ὤν (part. praes. к εἰμί).