Think of something that is meant to fit you and does not. A jacket can be beautiful on the and wrong the moment you put it on, because the question was never whether it is nice — it is whether it goes with you. Now hand the same question to an action: this is a fine thing for someone to do, but does it go with you, in the position you hold?
Wiktionary gives un- plus becoming, and compares Middle English unbicomelich; it also notes the parsing unbecome plus -ing. Becoming as an adjective is the positive partner, saying a thing does suit you.
Wiktionary 記載它由 un- 加 becoming 構成，並指出中古英語有對應的 unbicomelich，也提到可以看成 unbecome 加 -ing。becoming 當形容詞時是它的正面搭檔，意思是「襯得上你」；加上 un- 就成了「配不上」。
Wiktionary entry for unbecoming, packet source_ref kaikki:3f6f69a190036ae4:867637934
From the picture
做起來不合身分
把「合不合身」從衣服移到行為上。你現在穿的是一個身分——老師、軍官、法官、隊長——而某些舉動穿在這個身分上就是不對勁。英文用的還是同一個字，判斷的還是同一個「合不合」。這是概念上的連結，方便記，不是說一個意思在歷史上從另一個生出來。
adjective · C1
Behaviour that is unbecoming does not fit the position or role a person holds, so that it shocks people or makes them think less of that person.
不得體、有失身分
adjective · C1
Clothes, colours or hairstyles that are unbecoming do not suit the person wearing them and make that person look less attractive than usual.
不好看、不襯人