Genuinely upset or astonished and not just pretending to be so. From the term "shaken up" and professional wrestling, where the performers often pretend to be injured but are sometimes actually, legitimately injured.
"Steven was legit shook by the death of Ned Stark at the end of Game of Thrones, but Mike had read the books so he knew it was coming and was just saying "oh man" to tease Steve."
by DBrine July 3, 2013