Indeed, it’s one of these cases, where original meaning of the word was subverted and means something different than in popular usage.
‘Synchronisation’ in telecommunication means that receiver signals whenever it is ready to receive messages, and only after this signal the transmitter will start transmitting. When the transmitter is done with the message, it will signal it has finished, so that receiver can now process the received message and do whatever it is supposed to be doing next.
This is of course a simplification and a very broad one, but it should give you the feeling of from where the meaning of ‘(a)synchronous’ comes in JS.
So synchronous request in JS is actually synchronised with the main flow of the program. The program sends request to server (‘I’m ready to receive’) and waits for message. Message from the server will have a well defined end (‘the message ends here – do your job’). When it is received, JS knows it can continue with execution of the program..