To clack wool, to cut off the sheep's mark.--John Kersey's New English Dictionary, 1772
Feast Day of St. Dominic,
a patron of shepherds. John Spence's Shetland Folklore, (1899) mentioned an old custom from which the verb to earmark is derived: "Everyone knew his own sheep by the marks cut in their ears. The various sheep marks had names by which they were known, [such as] bits, crooks, fiddlers, and shöls indicating different cuts in the ear." In fact, a number of "shepherd's books" have long recorded markings in Britain, such as the one described in Thomas Ellwood's Glossary of Lakeland and Iceland (1895), "published at irregular intervals, containing the distinctive marks [from] districts of Cumberland, Westmoreland, and northern Lancashire. With the earmark and smit [marking with a soft red stone] together, the marks of upwards of six hundred farms or estates are given herein." T. Lewis Davies's Supplemental English Glossary (1881) added, "It seems to have been the custom for persons who could not write to make the same [design] with which they marked their sheep, do duty for their signature."
Thanks, Jeffrey, that's fascinating. ¬.¬