weakShe was too tired and weak to finish the race.
feebleMany of the pensioners were so feeble they had to be stretchered onto the ship.
punyHe was puny as a child.
weedyUKinformaldisapprovingI was just this weedy guy who always had his head in a book.
frailA frail, grey-haired lady walked slowly down the street.
ailingHe shared an apartment with his ailing mother.
sicklyHe was a sickly child.
defencelessUKHow could anyone hurt a small, defenceless child?
vulnerableThe disease most often affects vulnerable people such as children and the elderly.
faintIf you feel faint, sit down with your head between your knees.
fragileThey'd all had a few beers the night before and were feeling a bit fragile.