One way to hold this word: something is happening over there, and it never reaches you. No line runs from that thing to your heart, so nothing tightens in you and nothing pulls you towards it. Depending on which line is missing, you get the two uses: no line of worry, or no line of interest.
From the word’s history
Wiktionary analyses the word as un- + concerned, or unconcern + -ed. The un- is simply the switch that turns concerned off, which is why both uses are read against what concerned would have meant.
資料把這個字拆成 un- 加 concerned，或 unconcern 加 -ed。un- 就是把 concerned 關掉的開關。所以兩種用法都要對照 concerned 來理解：一邊關掉「擔心」，一邊關掉「有關、在意」。
Wiktionary entry supplied in evidence packet (source_ref kaikki:3f6f69a190036ae4:872662886): "From un- + concerned or unconcern + -ed."
登入就可以看每一支怎麼連回核心畫面。