【Weak】 , 【feeble】 ,  【frail】 ,  【fragile】 ,  【infirm】 ,  【decrepit】  can mean not strong enough to bear, resist, or endure strain or pressure or to withstand difficulty, effort, or use.

【Weak】 is by far the widest in its range of application, being not only interchangeable with all of the succeeding words but also capable of being applied where they are not. Fundamentally it implies deficiency or inferiority in strength; it may apply to the body, the will, the mind, or the spirit.

Often it implies a lack of power, skill, efficiency or ability to control. It may also suggest a sign of impairment of a thing’s strength (as a defect, a fault, or a dilution).

【Feeble】  not only is more restricted than  【weak】 in its range of application but also carries a stronger implication of lamentableness or pitiableness in that weakness. It is chiefly applied to human beings and their acts and utterances, then usually implying a manifest lack or impairment of physical, mental, or moral strength.

As applied to things,  【feeble】 implies faintness, indistinctness, impotency, or inadequacy.

【Frail】 , when it implies physical weakness, suggests not so much the impairment of strength as natural delicacy of constitution or slightness of build.

As applied to things the term usually implies liability to failure or destruction if the thing has physical existence or, if immaterial, an incapacity for dealing with forces or powers opposed to it, or tending to destroy it.

When  【frail】  is applied to the will, the conscience, the moral nature of man, it carries an even stronger implication of lack of power to resist than  【weak】 .

【Fragile】  (see also  【FRAGILE】 1 ) is frequently used in place of  【frail】 , but it usually carries even a stronger suggestion of delicacy and of likelihood of destruction.

【Infirm】 usually implies a loss of strength, especially of physical strength, with consequent instability, unsoundness, or insecurity.

As referred to human beings, it implies illness or more often old age. As referred, however, to the tempers, the designs, or the intentions of men, it often implies wavering or serious vacillation.

【Decrepit】 is as applicable to things as to persons that are worn out or broken down by use or age.