1st hypothesis: From Middle English Temese, from Latin Tamesis, ultimately of Celtic origin. The letter h was mistakenly inserted during the Renaissance, to make the word appear as if it was derived from Greek. The pronunciation, though, did not change to /θ/.
2nd: hypothesis: from an Anglo-Frisian origin; cognate with Amstel, Eems, Eem, all 3 rivers located in modern day Holland and Friesland, sharing geophysical features with the river Thames.
- (rivers in England and Ontario): enPR: tĕmz, IPA: /tɛmz/
Audio (UK) (file)
Audio (US) (file)
- (river in Connecticut): IPA: /θeɪmz/
Proper noun 
- River in southern England flowing 336 km (209 mi.) through London to the North Sea.
- River in Ontario province, Canada, flowing 258 km (160 mi.) to Lake St. Clair.
- Estuary in the U.S. State of Connecticut flowing 24 km (15 mi.) past New London to Long Island Sound.