adjective susceptible to attack
Similarassailablecompromisingdefenselessendangeredindefensibleinsecurepenetrablethreatenedunder attackunguarded
Oppositesinvulnerable
adjective susceptible to criticism or persuasion or temptation
Similarsusceptible
adjective capable of being wounded or hurt
Similarweak
Source: WordNet 3.1, Princeton University
