In English, there are many synonyms for the phrase "be deficient" which can be used to describe a lack of something or an inadequacy in something. Some of the most commonly used synonyms include words like "lack", "fall short", "fail to meet expectations", "inadequate", "deficiency", "shortage", "insufficiency", "ineffectiveness" and "unproductive". Each of these words can be used to describe a different type of deficiency, whether it is a physical or a mental shortcoming or an inability to perform a task. By using these synonyms, one can add depth and nuance to their writing and convey a more precise meaning to their readers.