"Spineless" and "Brainless" (and derivatives) are speciesist microagressions.
Body parts & configurations are amoral. Spines aren't strength and brains aren't intelligence not that either of those makes someone worth more or less anyway.
But lookup the decentralised nervous system (DNS) of the jellyfish for example who have no spines or brains.
These are just two more terms that reinforce human supremacy (those non-humans more similar to humans deserve more ethical consideration)
Also related insults: "Worm", "Insect", "Maggot"