E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Coystrel, or Kestrel. A degenerate hawk; hence, a paltry fellow. Holinshed says, costerels or bearers of the arms of barons or knights (vol. i. p. 162); and again, women, lackeys, and costerels are considered as the unwarlike attendants on an army (vol. iii. 272). Each of the life-guards of Henry VIII. had an attendant, called a coystrel or coystril. Some think the word is a corruption of costerel, which they derive from the Latin coterellus (a peasant); but if not a corruption of kestrel, I should derive it from costrel (a small wooden bottle used by labourers in harvest time). Vasa qudam qu costrelli vocantur. (Matthew Paris.)
Hes a coward and a coystril that will not drink to my niece.Shakespeare: Twelfth Night, i. 3.