Scare the pants off of is an informal idiomatic expression that typically means to terrify or intimidate someone. There are multiple synonyms for this phrase that can be used in different contexts. Some of the commonly used synonyms for scare the pants off of include petrify, spook, startle, frighten, alarm, and unsettle. Petrify is a word that denotes frightening someone so much that they become motionless or paralyzed with fear. Spook means to make someone feel scared or jittery. Startle refers to frightening someone suddenly with a quick and sharp action. Frighten denotes causing someone to feel scared or intimidated by something. Alarm refers to a sudden and intense fear of danger. Lastly, unsettle is a word that denotes making someone feel uncomfortable or anxious.