There are several synonyms for the word "under-perform." Some common alternatives include "underachieving," "falling short," "not meeting expectations," "failing to meet standards," and "lagging behind." These synonyms can be used to describe a wide range of situations in which someone or something isn't performing as well as they should be. For example, an employee may be underachieving if they frequently miss deadlines or produce subpar work, while a business may be falling short if it fails to meet its sales goals. No matter the circumstances, using synonyms for "under-perform" can help to convey the message clearly and effectively.