Since I’m on a roll with these name origins, and I want a relatively complete list for Norfolk, back to The Concise Oxford Dictionary Of English Placenames.
Great Yarmouth, Norfolk. Gernemwa in Domesday Book, Gernemuta Magna in 1254. The mouth of the River Yare.
The Magna means great, which is solely to avoid confusion with Southtown on the southern side of the river, formerly known as Little Yarmouth. The word origin is different from Yarmouth on the Isle of Wight, but the word Great has probably persisted to differentiate those two towns. ‘Garienis’ is the Saxon word for the River Yare, so it started with something like ‘Garmud’ and that ‘d’ at the end was pronounced as ‘th’.