Noun. A person who appears to be of average weight and size based on their face and upper body but who has an ENORMOUS lower half. The term comes from meeting someone at a table and speaking with them for a while thinking that they are a slender person (based on the face and upper body) but then getting a surprise when they stand up from the table and you see their large lower half for the first time - hence... "stand up surprise". The top/bottom differential must be extreme to count as a stand up surprise.

When you eat dirt for the first time and leave your family to build a shack in the woods