"Kick in the butt" is a common phrase used to imply motivation or encouragement to get something done. However, this phrase may not always be suitable in professional or formal settings. To express the same sentiment in a more polite manner, one may use synonyms like "nudge", "prod", or "push". These words convey a similar idea of motivating or inspiring someone to take action without sounding too aggressive or confrontational. Other alternatives may include "boost", "stimulate", or "galvanize". By choosing the right synonym, one can effectively communicate the desired message without offending or intimidating others.