stiff adverb greatly; used to intensify, especially “bore” and “scare” UK, 1905 From “stiff” (dead) hence, here, “to death”; always after the verb it modifies.
[H]itting little white balls bores him stiff. — NewYorkMetro, 14 December 1998
I appreciate Eamonn paying me the compliment of scaring me stiff. — Michael Palin, Sahara, 2002