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 quædom quæ costrelli vocantur.” (Matthew Paris.)
“He's a coward and a coystril that will not drink to my niece.” —Shakespeare: Twelfth Night , i.3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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