Unbecoming means improper for somebody of that rank or in that role. Conduct unbecoming, unbecoming of an officer, most unbecoming.
Conduct unbecoming is a formal military and professional charge, and the full phrase names the office: conduct unbecoming an officer. It measures behaviour against the position rather than against ordinary manners, which is why it can be serious.
1adjectiveunbecomingImproper for somebody of that rank or in that role.
“Shouting at the referee was behaviour unbecoming of a team captain.”
unbecoming is a quality.
Un- plus becoming, from become in its older sense of to suit or be fitting.
From un- + becoming. Compare Middle English unbicomelich (“unbecoming”). Equal to unbecome + -ing
Etymology from Wiktionary, CC BY-SA