When we say that someone didn't let out a peep, we mean they didn't make a sound, speak or give any indication of their presence. Some synonyms for this phrase include remaining silent, keeping quiet, staying mum, maintaining silence, and not uttering a word. Other expressions that can be used in place of 'not let out peep' include holding one's tongue, saying nothing, keeping one's mouth shut, and keeping one's lips sealed. These phrases all describe situations when someone chooses to remain completely silent, either out of fear, caution or simply being secretive. The use of these synonyms can add variety and depth to a writer's vocabulary and convey more precise meaning.