WordNet sense 1 (lacking a backbone or spinal column):

invertebrate

WordNet sense 2 (weak in willpower, courage or vitality):

gutless, namby-pamby, wishy-washy

WordNet sense 3 (having little physical or spiritual strength):

weak

WordNet sense 4 (lacking thorns):

thornless

WordNet sense 5 ((used of plants or animals) lacking barbs or stings or thorns):

unarmed