From Middle English sculle, scolle (also schulle, scholle), probably from Old Norse skalli ("bald head, skull"), itself probably related to Old English sċealu ("husk"). Compare Swedish skalle, Norwegian skalle.
Alternatively, perhaps from Old Norse skoltr, skolptr, akin to Icelandic skoltur, dialectal Swedish skult, skulle, Middle Dutch scolle, scholle, Middle Low German scholle, schulle. Compare also Old High German sciula, skiula.