These words refer to lack of strength or health or to an inability to bear strain or pressure. Weak is the most general of these, carrying no implications as to how the lack of strength came about. It may be used in a purely physical sense: weak and dizzy after a fainting spell; born with weak eyesight; walls that without buttressing would have been too weak to bear their own weight. It may refer to a lack of mental or moral strength, indicating instability of character or deficient will power: a weak youth, easily led astray by bad associates. Sometimes weak simply points to a lack of influence or authority: a government with a weak executive branch and a strong legislature. Generally, it may refer to any lack of normal power, strength or potency: a weak voice; weak coffee; a weak link in a chain.

Frail , when used of a person, stresses an extremely slender, delicate or sickly physique: frail and undernourished children who stared out from the windows of the tenement. In other situations, the word suggests something easily broken or unable to resist an opposing force: frail porphyry columns long since snapped in two; a frail theory that even the average person could refute. It is used colloquially to indicate a temporary unwell feeling, usually after too much to drink: a little frail after last night’s party. Infirm concentrates on a lack of soundness that is either inherent or that results from ageing, illness or the like: an infirm constitution inherited from his father’s side of the family; a mind that had grown infirm with poverty, sickness and old age. Used more abstractly, the word suggests a thoroughgoing faultiness resulting from incorrect methods of working: infirm conclusions based on deliberate distortions of the evidence. It may also point to a lack of stability or firmness, meaning irresolute or insecure: infirm of purpose; an infirm prop.

Feeble suggests a lack of strength that results in a fitful but always subnormal performance marked by a pitiable lack of alertness or resilience: a hand so feeble that it could scarcely lift the cup from its saucer; feeble-minded . When used of the human body, it generally suggests that a process of attrition has occurred. Used literally of things, it may mean faint or inadequate: a feeble light; a feeble cry; a feeble defence system. Used of abstract things or of ideas, it points to ineffectuality, pointlessness or pitiable performance: a feeble effort at courtesy; a feeble joke; a feeble interpretation of the play.

Debilitated and decrepit specifically suggest the sapping of strength formerly present. Debilitated by the ravaging disease; a house debilitated by its long exposure to the elements. Decrepit specifically restricts itself to a loss of strength or usefulness because of advanced age: decrepit dodders in the rest home; musty stairs grown so decrepit that they groaned under a child’s weight.