noun
merry drinker; person who gets cheerful when drinking
Describes someone whose mood becomes lively and happy after drinking alcohol. Often used in contrast with other 上戸 types like 泣き上戸 (crying drunk) or 笑い上戸 (laughing drunk).
彼は機嫌上戸で、お酒を飲むといつも陽気になる。
He's a merry drinker; he always becomes cheerful when he drinks.
機嫌上戸の人は、飲み会を盛り上げてくれる。
Merry drinkers liven up drinking parties.
Compound of 機嫌 (mood) and 上戸 (drinker; originally a term for someone who can hold their liquor well). The exact historical derivation is uncertain, but the word describes a type of drinker based on their mood change.