Mismetre

They neither maul nor mismetre his rhyme;

Source: wiktionary

For this he quotes and mismetres a fragment Kallimachos.

Source: wiktionary

From the evidence of other witnesses to alliterative texts copied by Thornton (as, for instance, by comparing the four dissimilar manuscripts of Awntyrs or the two each of Truelove and The Palement of Thre Ages) it can be seen that, although Thornton is a careful scribe and almost never mismetres a line into duple, alternating rhythms, he shows an eagerness to smooth out and add explanatory pleonasms by inserting fillers such as ful, frequently supplying full forms where an aphetic one was metrical, and altering the metrical pattern of the b-verses to make triple-beat 'anapaestic' lines.

Source: wiktionary