Lacking a backbone; without courage, determination, or strong moral principles; spineless.
From backbone plus -less suffix (meaning 'without'). The metaphorical use of backbone for character developed by the 1700s, making this term for moral weakness natural.
Some animals like jellyfish are literally backboneless and are called invertebrates, but calling a person backboneless is actually a pretty harsh insult meaning they're spineless and cowardly!
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