set the Thames on fire

The story

Unknown. Suggested to derive from a misconstrual of temse (“sieve”): thus, to work so vigorously as to heat a sieve by friction. Alternatively, a reference to lightning strikes which sometimes occurred along the Thames, occasionally setting trees on fire or causing death in unusual manner. Otherwise simply by hyperbole, from the improbability of setting a river on fire (although the image might sound like a familiar reference to the Great London fire of 1666).