WordNet sense 1 (pathetically lacking in force or effectiveness):

lame, lame

WordNet sense 2 (lacking strength or vigor):

faint

WordNet sense 3 (having little physical or spiritual strength):

weak

WordNet sense 4 (lacking physical strength or vitality):

decrepit, infirm, rickety, sapless, weakly, debile

WordNet sense 5 (physically weak):

frail

WordNet sense 6 (lacking strength):

nerveless

WordNet sense 7 (lacking power):

powerless