somebody doesn’t have much meat on him/her
From Longman Dictionary of Contemporary Englishsomebody doesn’t have much meat on him/hersomebody doesn’t have much meat on him/herBritish English, need some (more) meat on your bones American English informalTHIN PERSON used to say that someone looks very thin → meatExamples from the Corpusneed some (more) meat on your bones• Matt, you need some more meat on your bones!