Thames

From Middle English Temse, Temese, from Old English Temes, Temese (compare Welsh Tafwys), from Latin Tamesis, Tamesas. Variant spellings with h arose in Middle English due to the mistaken assumption of a Greek etymology. The modern spelling with a stems from either the East Saxon dialects of Middle English and Old English or the influence of Tamesis.

The Latin name is from Proto-Brythonic *Tamesis, from Proto-Celtic *tamesās (“river, waters”, literally “darkness”), a masculine ā-stem of *tames, Proto-Indo-European *tm̥Hes-, zero-grade of *témHes-, *témHos- (“darkness”), an s-stem from the root *temH- (“dark”). Related to Proto-Celtic *temeslos (“darkness”), *temos (“dark”). A parallel in Proto-Celtic of "dark, darkness" taking on the figurative meaning of "water" can also be found in Proto-Celtic *dubros (“water, dark”), from Proto-Indo-European *dʰubrós (“dark”), yielding Welsh dŵr (“water”), Irish dobhar (“water, sea, dark, gloomy”). Hydronyms with their origin in this term also occur elsewhere in Europe, for example Portugal's Douro.

Alternatively from Proto-Celtic *tā-, *tāyo- (“to melt, flow”), from Proto-Indo-European *teh₂- (“to melt”), or from unknown non-Indo-European root.

Possible cognates include the names of rivers and tributaries such as:

Taff, Tamar (from Latin Tamarus), Tame, Tavy, Team, Teifi (from Welsh Teifi, from Old Welsh Tebi, Teibi), Teme