From an unidentifiable account of early Roman history. ('Under the Fabii' presumably refers to alleged events of 480-474 BCE (Livy 2.44ff), when the men of Etruscan Veii occupied Rome's Janiculum hill and the Fabian gens countered by fortifying the Cremera -- where almost all of them were annihilated.) The headword is aorist passive of surrh/gnumi, third person singular: cf. sigma 1509.
[1] Greek idiom says 'greatest'.