The mustard was developed in the English town of Tewkesbury in Gloucestershire, and gained a certain reputation in the 17th century, becoming a staple condiment of the kitchens of the time.
Shakespeare mentions the mustard in Henry IV, Part 2, in which Falstaff has the line: “his wit's as thick as Tewkesbury Mustard” (Act 2, Scene 4, Line 244), describing the character of his friend Ned Poins.
There is some evidence that “Tewkesbury Mustard” came to be used as slang for incendiary 'fire-balls', a usage referred to in David Hume's History of England, in which he describes the rumour that the Great Fire of London was started by foreign arsonists trained by Jesuits "and the whole plan of operations was so concerted, that precautions were taken by the Jesuits to vary their measures, according to the variation of the wind.
[6] A similar line also appears in Alfred Marks' book Who Killed Sir Edmund Godfrey?.
1889) gives evidence that the phrase “He looks as if he lived on Tewkesbury mustard” came to be used as slang in Gloucestershire for those "who always have a sad, severe, and terrific countenance".