"Kick in the pants" is an idiomatic expression that denotes an action or situation that serves as a catalyst to motivate someone. There are several synonyms for this phrase that can be used depending on the context or intention of the speaker. Some of these synonyms include "wake-up call," "push," "nudge," "prod," "stimulus," "impetus," "incitement," "catalyst," "spark," "impulsion," "encouragement," "urge," and "prompting." Each of these synonyms has a slightly different connotation but all serve the same purpose of provoking and inspiring an individual to take action or improve their performance.