That of hir smylyng was ful symple and coy;
Hire gretteste ooth was but by seinte Loy,
(= Whose smiling was very innocent and quiet / Her greatest oath was only by Saint Elgius)
‘Coy’ has none of the modern, flirty connotations; the phrase was common in the French romances Chaucer will have read. This is a straight-up, ‘she was a nice kinda gal’. Not R. Kelly, in other words.
Nuns weren’t supposed to swear oaths or go on pilgrimages, but the Prioress has sworn a very mild one— St Elgius was a goldsmith, renowned for his courtliness and beauty, and his skill at metalwork.
It’s likely that no particular criticism is intended, and possibly even that the name was just chosen for the rhyme.
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