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.
prō-socer erī, m a wife's grandfather, O. ...
An Elementary Latin Dictionary