Notify the host application that a input event was not handled by the WebView. Except system keys, WebView always consumes input events in the normal flow or if shouldOverrideKeyEvent returns true. This is called asynchronously from where the event is dispatched. It gives the host application a chance to handle the unhandled input events. Note that if the event is a Android.Views.MotionEvent, then it's lifetime is only that of the function call. If the WebViewClient wishes to use the event beyond that, then it must create a copy of the event. It is the responsibility of overriders of this method to call WebViewClient.OnUnhandledKeyEvent(WebView, Android.Views.KeyEvent) when appropriate if they wish to continue receiving events through it.

[Android Documentation]