This entry is from the scholia to Homer, Odyssey 1.68, where the headword -- a neuter adjective used adverbially -- appears (web address 1 below).
The entry at Lexikon des frühgriechischen Epos I (fasc.8) 1404 derives the adjective from ske/llw 'dry up' with alpha copulative, and assumes a metaphorical history of "shriveled hard" similar to "steely." Perhaps compare "inveterate hatred." Numerous meanings and derivations have been offered since antiquity.