"Efyrnwy, ‘Fyrnwy, Vyrnwy. Ev(er)envy, Evernoy [= r. Gam] 1185, Blaen evernoe, Ewernoe [r. Gam] 1200, (r.) Ewernwy 1206-15, Evernow [r. Gam] 1226, Efyrnwy c.1400, 1543, Avernoye c.1485, i ‘Fyrnwy fawr 15c., Averney, Euerne, Murnwy fluvius c.1570 Evyrnwy 1588, Vurn way flu: 1578, Cantref Y Vyrnwy 1584, Forkt Vurnway 1612, vyrnowy, --noe, --noye, --oy 1588, Vyrniw 1650, the River Vyrnwey 1721, Fyrnwy, River Vyrnwy 1836. Fyrnwy and Y Fyrnwy to local W-speakers but modern printed sources tend to favour Efyrnwy.
The meaning is uncertain but is likely to contain a personal name *Ebur with a suffix -nwy. ebur appears to have meant 'yew -tree' and is later found in efwr 'cow-parsnip, hogweed'. Vyrnwy is now applied on Ordnans Survey maps solely to the river (formed by the joining of the Eiddo, the Nadroedd, the Eurnant and the Cedig in what is now Lake Efyrnwy/Vyrnwy reservoir) locally, Llyn Llanwddyn, which runs through Llanwddyn and Meifod to the river Severn/Hafren at Cymerau (SJ 327158). This is locally called Afon Pontlogel near Pontllogel and Afon Dolanog near Dolanog but Efyrnwy once also applied as a name to the rivers Gam and Banwy which meet the modern Efyrnwy/Vyrnwy near Mathrafal The two headwaters were sometimes distinguished as Efyrnwy Mechain and Efyrnwy Caereinion above the confluence with reference to the commote of Mechain and the cant ref of Caereinion through which they flow.