Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
prō-sŏcer, ĕri, m., a wife’s grandfather: cuique senex Nereus prosocer esse velit, Ov. H. 3, 74: uxor, et nurus, et pronurus, viro, et socero, et prosocero furtum facere possunt, Dig. 25, 2, 15: erum cum prosocero meo, Plin. Ep. 5, 14 (15), 8.