The headword (perhaps but not necessarily quoted from somewhere) is present infinitive of the doubly compound verb proe/gkeimai. Same glossing in Photius (pi1206 Theodoridis) and the Lexica Segueriana; Hesychius had glossed with prou+pa/rxein "to be before."
Outside lexicography (and epigraphy: see LSJ s.v.) this verb appears to be attested only in forms of the participle proegkei/menos.