coistril
An inferior groom or lad employed by an esquire to carry the knight's arms and other necessaries; a coistrel.
obsolete
A mean, paltry fellow; a coward.
obsolete
And he shall 'by his merriment as dear / As ever coistril bought so little sport: / Ne'er let this sword assist me when I need, / But rust and canker after I have sworn.
Where it comes from
Probably from Old French coustillier (“groom or lad”). Compare custrel.
Adapted from the Wiktionary entry coistril (CC BY-SA 4.0).