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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.