One way to hold this word: picture a space beside someone that is meant to be filled, and it is empty. A child walks to the gate and no adult walks with her. A suitcase goes round the belt and its owner is on another flight. A voice rises in a hall and no piano comes in under it. The word does not say the person is lonely or the music is poor. It only reports the empty place at the side.
From the word’s history
Wiktionary records the word simply as un- + accompanied. Accompany carries the idea of going along beside someone, so un- takes that companion away and leaves the space open.
維基詞典把這個字記成 un-（沒有）加上 accompanied。accompany 本來是「陪著一起走」，un- 把那個陪的人拿掉，於是旁邊就空了出來。這正好是今天各種用法共通的畫面。
Wiktionary entry for unaccompanied, kaikki:3f6f69a190036ae4:866169946:5846 — "From un- + accompanied."
登入就可以看每一支怎麼連回核心畫面。