Written only dash in apache access log
https://www.rfc-editor.org/rfc/rfc7231#section-6.5.7
6.5.7. 408 Request Timeout
The 408 (Request Timeout) status code indicates that the server did not receive a complete request message within the time that it was prepared to wait. A server SHOULD send the "close" connection option (Section 6.1 of [RFC7230]) in the response, since 408 implies that the server has decided to close the connection rather than continue waiting. If the client has an outstanding request in transit, the client MAY repeat that request on a new connection.
So, the client connected, but did not send any HTTP request. The server waited, and eventually closed the connection.