Fallowthought was one of Martin Pale's Fairy-servants; Gilbert Norrell described him as a "a very minor spirit" who probably did not have much influence over the Aureate magician. This servant was so little known he was not included in John Segundus's otherwise-exhaustive work, A Complete Description of Dr Pale's fairy-servants, their Names, Histories, Characters, and the Services they performed for Him.
Mr Segundus in fact only learns of the existence of Master Fallowthought when he visits Mr Norrell at his residence of Hurtfew Abbey in late 1806. Mr Norrell knew of Pale's fairy-servant from a book written by the magicians Holgarth and Pickle, in which their meetings with him were mentioned.
It is a little ill-natured of Mr Norrell to chide Mr Segundus for an omission he could not avoid, since the former must knows he himself has the sole copy of Curiose Observations. Scholars will have their little flings at one another.