Definition of "unwithstood"

As if their own indignant earth, / Which gave the sons of England birth, / Had felt their blood upon her brow, / And shuddering with a mother's throe, / Had turned every drop of blood, / By which her face had been bedewed / To an accent unwithstood, / As if her heart had cried aloud: […]

1819 (date written), Percy Bysshe Shelley, The Masque of Anarchy. A Poem. […], London: Edward Moxon […], published 1832, stanzas XXXV–XXXVI, pages 18–19