Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
‡ prō-măgister, tri, m., one who presides, rules, etc., in the place of another, a vice-president, vicegerent, vice-director (in class. Lat. pro magistro), Inscr. Fabr. p. 442, 49; Inscr Grut. 426, 5; 607, 1 al.