spineless has 5 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is spinous.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
weak in willpower, courage or vitality
lacking a backbone or spinal column
“worms are an example of invertebrate animals”
lacking spiny processes
“spineless fins”
Synonyms, antonyms and the broader and narrower relations for spineless come from Princeton University’s WordNet. Full sources and licences.
lacking thorns