The headword is the present active participle, masculine nominative (and vocative) plural, of the verb ska/llw, I stir up, hoe; see generally LSJ s.v. It is taken to be quoted from its earliest attestation, in Herodotus 2.14.2 (web address 1). There it is noted that the Egyptians living to the south of Memphis (Barrington Atlas map 75 grid E1) are able to produce crops without stirring up the soil; the river floods and then recedes, seed is sown over the wet ground, swine are let into the fields to tread the seed into the soil, swine herded out, and the farmers later harvest a crop.
[1] The gloss is the same form as the headword; see generally LSJ s.v. ska/ptw. Identical glossing in Hesychius (sigma819), the Synagoge (sigma103) and Photius' Lexicon (sigma269).