A heavy drinker; someone who can drink a lot or often drinks at social gatherings.
彼は上戸で、飲み会ではいつも最後まで飲んでいる。
He's a heavy drinker and always drinks until the end at parties.
上戸の友人は少量の酒では酔わないので、ペース配分が上手だ。
My friend, who can drink a lot, doesn't get drunk from a little alcohol and paces himself well.
Used as a suffix meaning ‘-drinker’ or someone with a particular drinking tendency (e.g., 飲み上戸 meaning someone who drinks readily).
彼は飲み上戸で、集まりでは率先して乾杯するタイプだ。
He's the type who readily drinks at gatherings and often initiates toasts.
Ryukyuan-dialect usage: a suffix meaning a lover/fan of something (e.g., ～lover, ～-phile).
彼は映画上戸（じょーぐー）で、週に一度は映画館に行く。
He's a movie buff (in the Ryukyuan sense) and goes to the cinema once a week.