jean-paul buquet • Link
The OED gives as meaning 3 of "coxcomb" the following definition:
3. A fool, simpleton (obs.); now, a foolish, conceited, showy person, vain of his accomplishments, appearance, or dress; a fop; a superficial pretender to knowledge or accomplishments (…)
along with the following Pepys quote:
(…) 1667 Pepys Diary (1879) IV. 236 A vain coxcomb he is, though he sings and composes so well.
cumgranosalis • Link
as above plus Samuel Johnson : refs King Lear
then " fop; a superficial pretender to Knowledge or accomplisments."
refs: Hudibras p 1 cant iii.;
L'Estrange Fable 135:
Chart showing the number of references in each month of the diary’s entries.