feeble or weak in body or health, esp. because of age; ailing.

unsteadfast, faltering, or irresolute, as persons or the mind; vacillating

infirm of purpose.

not firm, solid, or strong

an infirm support.

unsound or invalid, as an argument or a property title.

to invalidate.

adjective

firm , healthy , hearty , robust , sound , strong

adjective

ailing , anemic , anile , debilitated , decrepit , delicate , enfeebled , failing , faint , faltering , feeble , flimsy , fragile , frail , halting , ill , insecure , irresolute , laid low , lame , sensile , shaky , unsound , unstable , unsubstantial , vacillating , wavering , wobbly , insubstantial , puny , weak , weakly , precarious , tottering , tottery , unsteady , unsure , brittle , disabled , doddering , sickly , worn