One way to hold this word: something that should have a hard part in it does not. English calls two hard things a spine — the row of bones that holds a body up, and the sharp point on a plant. Take either away with -less and you get a thing that cannot hold itself up, or a thing that cannot hurt you. Both of those pictures are still visible when the word is aimed at a person, which is why calling someone spineless says both at once: nothing inside holding them up, and nothing there to push back with.
From the word’s history
The word is simply spine + -less, so its meaning is assembled in front of you rather than inherited from anywhere older.
這個字就是 spine 加 -less 組起來的，意思是當場拼出來的，不是從更古老的字傳下來的。所以三個用法其實只是「spine 指哪一種硬東西」的差別。
Wiktionary entry for spineless (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1168760707): "From spine + -less."
登入就可以看每一支怎麼連回核心畫面。