599 Network Connect Timeout Error
Meaning
A non-standard status code used by some HTTP proxies to indicate that a network connection to the upstream server timed out (i.e., the proxy could not establish a connection to the origin).
When to Use
When a proxy tries to connect to an upstream server but fails to establish the connection within the timeout period, it may respond to the client with 599 to signal that the connection attempt timed out.
Example
If a proxy server cannot connect to the backend (for example, due to network issues or the server not listening) and the attempt times out rather than immediately refusing, the proxy might return a 599 Network Connect Timeout Error to the client.
When Not to Use
Do not use as a normal status code in applications. This is mostly used internally by certain proxies or client libraries to represent a connection timeout. Standard HTTP does not define 599, and typically connection timeouts would result in no HTTP response at all or a 504 if one wanted to signal a gateway timeout.
Source
Unofficial (HTTP proxy)