hsb.horse
It combines timestamp, SAPISID cookie, and origin into a SHA-1 based header value to strengthen request validation.
Practical Note
SAPISIDHASH usually appears in contexts related to security, authentication, google, api. In practice, it helps to know not only the definition, but also what this term is trying to name quickly in a conversation, design note, or document.
Nearby words often overlap and make the explanation fuzzy. It is easier to use the term well when the target, role, and typical situation are kept one step more concrete.
Reading Note
The easiest way to read this term is to look at three things first: what it is about, what nearby concept it should be separated from, and what kind of decision it usually supports. For SAPISIDHASH, the security, authentication, google, api context is already a good starting point.
It also helps not to stop at the definition alone. The more useful view is to see what the term is trying to name quickly inside a working conversation.