Perhaps from prason (a leek, and so an onion-patch); a garden plot, i.e. (by implication, of regular beds) a row (repeated in plural by Hebraism, to indicate an arrangement) -- in ranks.
Thayer's Greek Lexicon
Strong's 4237: πρασιά
πρασιά, πρασιας, ἡ, a plot of ground, a garden-bed, Homer, Odyssey 7, 127; 24, 247; Theophrastus, hist. plant. 4, 4, 3; Nicander, Dioscorides (?), others; Sir. 24:31; ἀνέπεσονπρασιαίπρασιαί (a Hebraism), i. e. they reclined in ranks or divisions, so that the several ranks formed, as it were, separate plots, Mark 6:40; cf. Gesenius, Lehrgeb., p. 669; (Hebrew Gram. § 106, 4; Buttmann, 30 (27); Winer's Grammar, 464 (432) also) § 37, 3; (where add from the O. T. συνήγαγοναὐτούςθημωνιαςθημωνιας, Exodus 8:14).
Mark 6:40 - N-NFP GRK:καὶ ἀνέπεσαν πρασιαὶ πρασιαὶ κατὰ NAS: They sat down in groups of hundreds KJV: they sat down in ranks, by INT: And they sat down groups [by] groups by
Mark 6:40 - N-NFP GRK:ἀνέπεσαν πρασιαὶ πρασιαὶ κατὰ ἑκατὸν INT: they sat down groups [by] groups by hundreds