ワンマン

わんまん
noun, na-adjective
one-man, autocratic, solo-operated
1. autocratic, dictatorial
Describes a person, especially a boss or leader, who makes all decisions alone without consulting others.
ワンマン社長(しゃちょう)だ。
He's an autocratic company president.
ワンマン経営(けいえい)社員(しゃいん)ついてこない。
With autocratic management, employees won't follow along.
Everyone is dissatisfied with his dictatorial attitude.
2. one-man, solo-operated
Operated by a single person, especially buses or trains that run without a conductor.
ワンマンバス()
I ride a one-man bus.
This line operates with one-man (driverless conductor) service.
Local buses mostly run as one-man operations.

ワンマン is wasei-eigo from English "one-man." It has two distinct uses in Japanese.

SENSE 1 — AUTOCRATIC:
Describes a leader who monopolizes decision-making. Almost always negative.

SENSE 2 — SOLO-OPERATED:
Refers to public transport operated by a driver alone, without a separate conductor.

Usage

In English, "one-man" is neutral, but in Japanese, sense 1 carries a strongly negative nuance. Sense 2 is a neutral, technical term used in transportation.