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.
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."
From the picture
把畫面放到人身上
把「沒有那根撐住的骨頭」搬到性格上。身體沒有脊椎就撐不住自己，人沒有骨氣就在壓力下折了——這是個教學上的連結，用來記這個字，不是說它歷史上先有哪個意思。
adjective · B2
Not brave enough to do what is right or to stand up to other people. Someone spineless gives in, goes quiet, or backs away at the moment when it would take courage to act, and the word shows that the speaker has no respect for that.
沒骨氣的；懦弱的
adjective · C1
Having no backbone. It describes animals such as worms, jellyfish and insects, whose bodies hold together in some other way.
無脊椎的
adjective · C1
Used about plants: having no sharp spines, prickles or thorns. A spineless cactus can be picked up without hurting your hand.
（植物）沒有刺的