Banw 1576, Banwy 1796, 1811, River Banw 1836, Banno c.1840. The river-name probably means 'pig' or 'young pig', probably because of its burrowing or rooting nature. The ending -y has probably been added in late sources under the influence of the river Efyrnwy/Vyrnwy but Banw is the form in daily use. Banw properly applies to the river down to Pont Glanbanw (SJ 084079). Below this point, modern Ordnans Survey maps name the river Banwy neu [or] Einion but earlier historical sources show that this stretch was once known as part of Efyrnwy sometimes described as Efyrnwy Caereinion because it flows through the cantref of Caereninion. Einion is a false form drawn from a supposition that the name Caereinion contained a river-name with care 'fort'.