The earliest recorded form of the name, as found in the
Book of Llandaf (
c.1120s), is
merthir issiu. This is the Welsh word
merthyr ('burial site, shrine, church') and what is probably a personal name which in modern Welsh would be
Isiw or
Isw.
Melville Richards gives
Merthyr Isw as a modern form of the parish's name. In 1555 the name appears as
Llanysho. In this case, the element
llan ('enclosure, church') has replaced
merthyr, as has happened in a number of similar names. The name appears as
Pertrissw in the list of parishes in
National Library of Wales,
Peniarth MS.147 (
c.1566). The modern English form is now
Partrishow. In Welsh, the form
Llanisw and
Merthyr Isw may be found. ==History and amenities==