The phrase 'be limp' refers to the state of something that lacks firmness and strength. It can describe people, objects, or even emotions. There are several synonyms that can be used to convey a similar sentiment. 'Slack', 'floppy', and 'lifeless' all describe something that lacks rigidity and vitality. 'Flaccid' and 'limber' can be used to describe a lack of tension or firmness in muscles. 'Languid' connotes a lack of energy or vigor, while 'listless' implies a lack of enthusiasm or zest. All of these synonyms can be used to effectively describe something that is limp, lifeless, or lacking strength.