Rare term for the back of the body; modern Japanese normally uses 背中 (せなか) or 背 (せ).
In old documents, there are examples where 背甲 is written and read as せごう.
The kanji 背 means 'back' and 甲 can mean 'shell' or 'armor', but the specific derivation of 背甲 as a rare reading for 'back' is uncertain.