Definition of "fierceness"

I stroked it gently, on which the mutinous rogue seemed to swell, and gather a new degree of fierceness and insolence; so that finding it grew not to be trifled with any longer, I prepar'd for rubbers in good earnest.

1749, [John Cleland], “(Please specify the letter or volume)”, in Memoirs of a Woman of Pleasure [Fanny Hill], London: […] G. Fenton [i.e., Fenton and Ralph Griffiths] […]

Travellers over the London & North Western main line in bygone days will need no reminder of the pattering of cinders on the carriage roofs, the fountains of sparks from the chimneys at night and the distance from which the exhaust of approaching locomotives could be heard, due to the fierceness of their blast in such conditions.

1962 June, Cecil J. Allen, “Locomotive Running Past and Present”, in Modern Railways, page 399