Same or similar entry in other lexica. The headword is present participle, nominative masculine singular, of the verb e)peuwni/zw. It must be quoted from somewhere; Theodoridis on Photius s.v. suggests Basil of Caesarea (PG 29.269b) but there are several other possibilities (in e.g. Plutarch, Philo Judaeus, Cassius Dio).
The gloss assumes a (common-sense) principle that lowering the price increases sales volume.