The phrase "weak jaws" refers to someone who has a lack of determination or strength to handle difficult situations or stand up for themselves. Full definition
Researchers, however, were able to find that humans can bite slightly harder despite having weaker jaw muscles. (techtimes.com)
The good news is you can start addressing it in a young, hopefully plastic, spongy puppy with weak jaws. (dogsnaturallymagazine.com)
Compared with chimps, humans have evolved weak jaw muscles and jaw bones — possibly because social organisation reduced the need to bite as a form of attack (newscientist.com)