Someone who's defenseless is vulnerable and unprotected. Babies need to be taken care of because they're completely defenseless.
A brand new chess player is somewhat defenseless against an experienced player, and a kitten is defenseless without its mother's fierce protection. Con men sometimes target people they see as defenseless, like the very elderly. The word defenseless dates from the 16th century, combining defense, "action of guarding or protecting," from the Latin defendere, "protect," and -less, "without or lacking."