(1 syl.). The Latin Thamesis (the broad Isis, where isis is a mere variation of esk, ouse, uisg, etc., meaning water). The river Churn unites with the Thames at Cricklade, in Wiltshire, where it was at one time indifferently called the Thames, Isis, or Thamesis. Thus, in the Saxon Chronicle we are told the East Anglians “overran all the land of Mercia till they came to Cricklade, where they forded the Thames.” In Camden's Britannia mention is made of Summerford, in Wiltshire, on the east bank of the “Isis” (cujus vocabulum Temis juxta vadum, qui appellatur Summerford). Canute also forded the Thames in 1016 in Wiltshire. Hence Thames is not a compound of the two rivers Thame and Isis at their junction, but of Thamesis. Tham is a variety of the Latin amnis, seen in such words as North-ampton, South-ampton, Tam-worth, etc. Pope perpetuates the notion that Thames = Thame and Isis in the lines -
Around his throne the sea-born brothers stood; Who swell with tributary urns his flood: First the famed authors of his ancient name, The winding Isis and the fruitful Thame! The Kennet swift, for silver eels renowned; The Loddon slow, with verdant alders crowned; Cole, whose dark streams his flowery islands lave; And chalky Wey that rolls a milky wave; The blue transparent Vaudalis appears; The gulphy Lee his sedgy tresses rears; And sullen Mole that hides his diving flood; And silent Darent stained with Danish blood.
He'll never set the Thames on fire. Hell never make any figure in the world; never plant his footsteps on the sands of time. The popular explanation is that the word Thames is a pun on the word temse, a coru-sieve; and that the parallel French locution He will never set the Seine on fire is a pun on seine, a drag-net; but these solutions are not tenable. There is a Latin saw, “Tiberim accendere nequaquam potest, ” which is probably the fons et origo of other parallel sayings. Then, long before our proverb, we had “To set the Rhine on fire” (Den Rhein anzünden), 1630, and Er hat den Rhein und das Meer angezündet, 1580.
“There are numerous similar phrases: as ‘He will never set the Liffey on fire;’ to ‘set the Trent on fire;’ to ‘set the Humber on fire;’ etc. Of course it is possible to set water on fire, but the scope of the proverb lies the other way, and it may take its place beside such sayings as ‘If the sky falls we may catch larks.’”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894