(as) weak as a kitten
Especially weak, feeble, and vulnerable; completely unable to defend oneself. The heart attack left me bedridden and weak as a kitten for nearly a month. Tom's always been as weak as a kitten, so we don't bother trying to roughhouse with him at all.
weak as a kitten
Feeble and fragile, as in After that bout with flu she was weak as a kitten. This simile has largely replaced weak as a cat, from the early 1800s.
weak as a kitten
Feeble, defenseless. This proverbial simile goes one better than weak as a cat, dating from the early nineteenth century. Erle Stanley Gardner had “He felt as weak as a kitten” in The Case of the Drowsy Mosquito (1943). Other variations are O. Henry’s weak as a vegetarian cat (Cupid à la Carte, 1907) and weak as a newborn kitten.