These words refer to something open to attack or left unprotected from possible harm. Vulnerable , the most wide-ranging of these, comes from a Latin root meaning to wound; reflecting its derivation, the word can literally refer to the danger of physical wounding: the boxer’s lowered guard left him vulnerable to his opponent’s jab. Vulnerable always stresses a lack of protection against physical or mental harm: an uncritical admiration of her husband that made her vulnerable to his every whim. The word is relevant also to the context of argument, where it applies to an assertion that cannot be supported or corroborated or that is easily rebutted: a new foreign-policy statement that was vulnerable to attacks from both the Left and the Right. In military strategy, it suggests a position that is liable to capture or encroachment by the enemy: attempts to lure the guerrillas into the open countryside where they would be most vulnerable to air attacks; a vulnerable observation post.

Defenceless refers strictly to an undesirable inability to ward off harm or danger. In this sense it is more extreme than vulnerable , since it suggests an utter lack of protection or precaution, whereas vulnerable can suggest defences inadequate to some danger or threat.

• The most massive defence system would still leave the nation vulnerable to nuclear attack, though it might comfort the average citizen to think that he was not defenceless.

Defenceless adds also a note of complete helplessness: a fenceless child. Exposed emphasizes a lack of protection that might shield one from discomfort, harm or danger: broken windows that left their them exposed to the sub-zero temperatures; a battle plan that left their left flank dangerously exposed to attack. Sometimes the word can suggest an actual testing of something, whether or not the possibility of harm is involved: experiments in which rabbits were exposed to the drug as a way of checking its suitability for use by humans. Sometimes it is the risk rather than the harm that is emphasized: an attempt to track down everyone who had been exposed to the bacillus; his odd ideas, which left him exposed to ridicule.

Untenable reflects its derivation from a French word in referring to what cannot be "held" ?that is, defended or maintained. In the context of argument, the word refers not only to a statement that may be refuted, as in the case of vulnerable , but to one that is inherently unsound.

• Although the prosecution’s case was vulnerable , the defence had chosen a line of reasoning that was inept and untenable .

In terms of military strategy or, by extension, any form of competition, the word suggests a precarious position or deployment of forces that cannot resist attack: an untenable beachhead, exposed to attack from high ground on all three sides. The word suggests a greater certainty of defeat than vulnerable and almost as much as defenceless .