See Geiriadur Prifysgol Cymru/A Dictionary of the Welsh Language sub maerdref although maerdy is also possible. The Anglo-Norman attestation seems to use a mixed form of both (semantically overlapping) words.
s.
1agricultural(dairy) farm subject to a lord, in Wales:
(
c.1264-79
)
les vaches e les genyces e lé bovet (l. boves) mist a son meyrdif de Hoptoun en son estor
482
meyrdif (c.1264-79)