WordNet sense 1 (retarded in intellectual development):

feebleminded

WordNet sense 2 (moving or directed or tending in a backward direction or contrary to a previous direction):

retrograde

WordNet sense 3 (directed or moving toward the rear):

reverse, rearward

WordNet sense 4 (in a manner or order or direction the reverse of normal):

backwards

WordNet sense 5 (at or to or toward the back or rear):

back, rearwards