One way to hold this word: picture two sides arguing inside a ring, and one person standing outside it. Nothing in the result is theirs — no money, no job, no friend to protect — so nothing pulls their judgement one way. The dis- is not about being bored; it is about having no share in the fight. Two readings grow out of that picture. In the first, standing outside is exactly what makes the judgement worth having. In the second, readers take dis- plus interested at face value and hear 'not interested' — a reading many teachers and editors still push back on.
From the word’s history
Wiktionary gives two routes for the form: from the verb disinterest plus -ed, or from dis- plus interested, where interested means 'having a stake in'. Both routes land on the same idea — no stake in the outcome.
Wiktionary 給了兩條來源：一條是動詞 disinterest 加 -ed，另一條是 dis- 加 interested，而這裡的 interested 指的是「有利害關係的」。兩條路都走到同一個意思：對結果沒有份。
Wiktionary, 'disinterested' (adj), supplied in evidence packet (source_ref kaikki:3f6f69a190036ae4:1079562906)
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