If you're spineless, you're weak or cowardly — or both. It would be spineless to watch a bully pick on someone much smaller and not try to intervene.
You can certainly use this adjective to literally mean "lacking a spine," and you might if you're talking about invertebrates like the spineless forest lizard or the Garo spineless eel. It's more common to call people spineless as a derogatory way of commenting on their weakness. A spineless superhero is too scared to confront the bad guys, and a spineless politician avoids conversation with critical constituents.