Ignorance, madam, pure ignorance.
Source: His reply on being questioned, by a lady reader of his Dictionary, why he had incorrectly defined 'pastern' as the 'knee' of a horse. Life of Johnson, (J. Boswell), Vol. I
-- Samuel Johnson, (Sep 18 1709-1784), English lexicographer, critic; He was remembered for writing the first critique of Shakespeare, 1765 and Dictionary of the English Language, 1755.