selfish; self-centered; acting alone without consulting others
Describes someone who does things entirely on their own, ignoring others' opinions or convenience. Often used critically. Cross-referenced with 自分勝手, which is a more common synonym.
See also: 自分勝手
Everyone is fed up with his selfish behavior.
Don't decide on your own; please discuss it with us properly.
More common and broadly used for selfishness. 一人勝手 emphasizes acting alone without consulting others, while 自分勝手 focuses on self-centeredness regardless of whether others are involved.
Compound of 一人 (ひとり, 'one person, alone') and 勝手 (かって, 'one's own convenience, selfishness'). The exact historical derivation is uncertain, but the combination clearly conveys acting solely according to one's own wishes.