Hang heavy drapes over the window, and keep them closed. Heavy fabric absorbs sound waves.
Install sound-deadening plastic film to the panes on the inside of the window. This plastic film reduces the ability of the glass to vibrate, thereby reducing transmission of sounds.
Plant shrubbery in front of and in the area immediately around the window in question. Shrubbery will absorb some noise before it reaches the window.
Grow ivy or another climbing vine on the exterior wall containing the noisy window. The mass of ivy leaves and stems will deaden noise as it reaches the wall. If you do not trim the vines from in front of the window, they will absorb additional sound waves.
Plant deciduous shade and evergreen trees to surround the area with their sound-deadening mass. Although deciduous trees lose their leaves in winter, the mass of the branches in the bare canopy will absorb noise.