Late 18th century; earliest use found in James Woodforde (1740–1803), diarist and Church of England clergyman. Probably partly from mash + potato, and partly shortened from mashed potato.
Definition of mash potato in US English:
mash potato
nounˌmaʃ pəˈteɪtəʊ
US
1Mashed potato.
2Also with capital initials. = "mashed potato".
Origin
Late 18th century; earliest use found in James Woodforde (1740–1803), diarist and Church of England clergyman. Probably partly from mash + potato, and partly shortened from mashed potato.