WordNet sense 1 (affected with, suffering from, or characteristic of rickets):

rachitic

WordNet sense 2 (affected by an impairment of normal physical or mental function):

ill, sick

WordNet sense 3 (inclined to shake as from weakness or defect):

shaky, wobbly, wonky

WordNet sense 4 (lacking stability or fixity or firmness):

unstable

WordNet sense 5 (lacking physical strength or vitality):

decrepit, feeble, infirm, sapless, weak, weakly, debile

WordNet sense 6 (physically weak):

frail