A normally calm person who becomes suddenly enraged or violent.
Origin
Early 19th century; earliest use found in Oriental Herald. From French mouton enragé, lit. ‘angry sheep’ from mouton + enragé, past participle of enrager.
Definition of mouton enragé in US English:
mouton enragé
nounˌmuːtɒn ˌɒnraˈʒeɪ
A normally calm person who becomes suddenly enraged or violent.
Origin
Early 19th century; earliest use found in Oriental Herald. From French mouton enragé, lit. ‘angry sheep’ from mouton + enragé, past participle of enrager.