Heard on the MSNBC show with Chris Hayes. This is a conflation of “scared stiff” (utterly terrified) and “worried sick” (very concerned about a person or situation). I have heard this one a lot. “Sick” and “stiff” are similar sounding words, contributing to the mashup. A big thanks to Frank King for hearing this one!
If you liked this one, check out my book on malaphors, “He Smokes Like a Fish and other Malaphors”. It’s available on Amazon for a cheap $7.99. Just click on the link – https://www.amazon.com/dp/0692652205