to and fro
adverb [ before noun ], adjective uk/ˌtuː ən ˈfrəʊ/ us/ˌtuː ən ˈfroʊ/
往复地；来回地
in one direction and then in the opposite direction, a number of times
She was gazing out the window, rocking rhythmically to and fro. 她注视着窗外，身体有节奏地来回摇晃着。