Similar word to infirmest:

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