The name is entirely obscure. The word is an alternative of 'chomp', to crush or mash up, a usage known at least since the time of John Bradford, Dean of St Paul's, who gave a sermon in 1574; "he wyll broch you and eate you, chaw you and champ you.". 'A dialogue in the Devonshire dialect', 1837 gives; "Chump, v. a. to masticate audibly, corr. from Champ"
The term is known in Scotland in the early 19th Century as a synonym for 'mire' (Jamieson's 'Supplement to the Etymological dictionary of the Scottish language', 1825) and is used as part of the definition of the word 'Brúitín' or 'Brutheen' meaning 'a thing mashed-up' in William Carleton's 1830 'Traits and stories of the Irish peasantry'; "Brutheen is potato champed with butter. Any thing in a loose, broken, and irregular state, is said to be in brutheen — that is, disorder and confusion".
Original Receipt in 'A modern system of domestic cookery, or, The housekeeper's guide' by M Radcliffe. (Radcliffe 1822)Potatoes mashed with Onions.
Prepare some boiled onions, by putting them through a sieve, and mix them with potatoes. In proportioning the onions to the potatoes, you will be guided by your wish to have more or less of their flavour.