Strengthen or reinforce (4,2)
I believe the answer is:
beef up
I believe this is a double definition.
'strengthen' is the first definition.
(synonyms)
'reinforce' is the second definition.
(thesaurus)
(Other definitions for beef up that I've seen before include "buttress" , "Give (something) more strength" , "Give added strength" , "Add strength to" , "message poor meat-eaters won't want to hear?" .)