firigend-strēam, m.n: a mountain-stream, the ocean. (“fee-ree-yend-streh-am”)
From the British Library medieval manuscripts blog: Detail of a miniature of a siren from a Bestiary, with extracts from Gerald of Wales on Irish birds. England (Salisbury?), 2nd quarter of the 13th century. Harley 4751, f. 47v. The siren has seized a ship and destroyed its mast. One of the sailors is trying to close his ears to her horrible song.