Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
rē̆trō-grădĭor, di, v. dep. n. (collat. form rē̆trōgrădo, āre, v. n., Mart. Cap. 8, § 887), to go back or backwards, to retrograde, Plin. 8, 15, 16, § 39; of the retrograde motion of the stars, id. 2, 15, 12, § 61; 2, 17, 14, § 76; Mart. Cap. 8, § 883.