The surname Sleath with its variants Slay, Slee, Sleith, Sligh and Sly, is derived from the Middle English sleg or Old Norse sloeg-r, meaning "clever, cunning", hence "sly". The original meaning of the Norse word was "able to strike", slog- being the preterite stem of the verb sla "to strike".
In the days when communities were small a person was identified by a single name only. With the increase in, and movement of population, confusion arose and it became necessary to adopt an additional cognomen, which was coined from one of four sources - the name of ancestor, a place (usually of origin or residence), an occupation or some personal characteristic or nickname. Thus, a man named John who was clever or cunning might be known as "John (the) sly" in order to distinguish him from others of the same Christian name. In the course of time the cognomen became hereditary in what we now term surnames, ceasing to have any reference to the bearer's personal characteristics.
The Middle English word became Slee in the north and Sligh or Sly in the south and midlands, which accounts for the various forms of the modern surname. Sleath, Sleeth and Sleith are the result of the pronunciation of gh as th which also occurs in the Yorkshire place-name Keighley.
A hundred years ago an average of ten births per year in this name is recorded in the indexes of the General Register Office. Of the fifty-one recorded between 1860 and 1864,eight are inevitably in the environs of London and are almost certainly due to migration, and no less than fourteen are within the county of Leicester; of the rest all but three are from the small area east and north-east of Leicestershire. Seven Sleaths, freeholders, men of some small property, voted in the 1830 election for knights of the shire in Leicestershire: they lived at Sapcote, Barwell, Hinckley, Leire, Rothley, Grimston and Thurnby. In the early eighteenth century Sleaths were living and dying and making their wills in the districts of Gilmorton, Husbands Bosworth and North Kilworth.
Early records mention Walter Sleh who appears in the 1219 Feet of Fines of Essex; Thomas Sleh, Slei or Slegh, in the 1219 Assize Rolls of Lincolnshire; Robert Sley, in the 1221 Assize Rolls of Warwickshire; John le Slege, in the 1273 Hundred Rolls of Oxfordshire; Ralph Sly, in the 1273 Hundred Rolls of Huntingdonshire; John Slygh, in the 1327 Subsidy Rolls of Sussex; and John le Slegh, in the 1333 Register of the Freemen of York.