The first occurrence of the surname has been traced to the part of England now called Cumbria, at the start of the 19th Century. Aided by DNA analysis, a common ancestor has been identified, from whom all living Had(d)aths are believed to be descended. The surname appears to have developed as a variant of Haddow. Research is currently stuck at 1767 when William Haddow of Pennington (one mile west of Ulverston) married Agnes Boulton of Baycliff (on the coast, three miles south of Ulverston) at Aldingham-in-Furness Parish Church (on the coast, one mile south of Baycliff) on 21st February.

The origin of the name is believed to be the Middle English for 'half', plus the Gaelic dabhach, which is a measure of land equivalent to four ploughgates (so the name means 'two ploughgates'). A plough worked by eight oxen was capable of bringing 104 acres into tillage in a year. A ploughgate was therefore the name that was given to 104 acres of arable land, and a 'half dabhach' or 'hadabhach' (being half of four ploughgates) is therefore 2 x 104 = 208 acres of land - hence the title of the Had(d)ath Family History book.