scare (one) stiff
To shock or frighten someone very suddenly or severely. Don't sneak up on me like that, you scared me stiff! That car accident seems to have scared Janet stiff—she's still shaken by it.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
scare someone stiff
Fig. to frighten someone severely. (Stiff = dead.) That loud noise scared me stiff. The robber jumped out and scared us stiff.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.