The phrase "scare the pants off" is a colloquial expression used to mean frighten someone severely. There are several synonyms that can be used in place of this phrase. For example, one could use the phrase "terrify" or "petrify" to convey a similar meaning. Alternately, "spook" or "startle" may be used to describe a milder degree of fear. Other options could be "frighten", "alarm", or "shock". Each word emphasizes a different level of intensity, so it is important to choose the appropriate word depending on the situation and context in which it is used.