Two places in Britain are called identically Foulness, but they are pronounced differently, denote different geographical features, and have different etymologies.
- The river Foulness, which runs roughly north-south and enters the Humber estuary, via the Market Weighton canal, at Brough, is pronounced 'FOOl-nay' (IPA: /'fuːl neɪ/), with the '-l-' more or less suppressed. It takes its name - and pronunciation - from its Old English root fule[n] ēa, 'dirty water', probably from the staining caused in it by the rust from iron deposits.
- The island of Foulness in Essex, on the Thames estuary, is pronounced like the common noun foulness, but with the stress on the second rather than the first syllable: the island is 'foul-NESS', /faʊl 'nɛs/; the common noun 'FOUL-ness', /'faʊl nɪs/. The name of the island was fughelnesse in Old English, 'headland of fowls [= birds]'. It shares this derivation with the Shetland island of Foula and the Faroese island of Fugloy (Fuglø in Danish), all being descended from the Old Norse Fuglaey.
The name "Foula" derives from Old Norse Fuglaey, "bird island": compare the Faroese name of the island of Fugloy, "bird island". In Gaelic it is Fughlaigh.
- The common noun is simply 'the state of being foul', or 'dirtiness'.