If you are defenceless, you cannot protect yourself. You have no wall, no weapon, no one to stand in front of you. People use it for a person, an animal, or even a place: a defenceless child, a defenceless village. It often carries feeling — the speaker thinks this weakness is unfair or sad, and that someone stronger should not hurt them. It can be literal (no army, no lock) or a little stretched (defenceless against gossip, against illness). This spelling with c is British, from defence. American writing uses defenseless, with an s. Same meaning, different letter.

毫無防備的；無力自衛的