From Hull AWE
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]'.
- The common noun is simply 'the state of being foul', or 'dirtiness'.