A dab hand is an expert and the expression has been known and used in this sense since the 17th century. Although the date of origin is well attested, the etymological origin remains obscure. There was some excitement when a printer’s tool for applying ink, called a dab or dabber, was put forward as the origin because the user of this tool was called a dab hand. Unfortunately, the expression dab hand was in use long before the appearance of the printer’s dab. Printers were merely making use of an expression that already existed. This is what the OED makes of it all. “Appears before 1700; frequently referred to as school slang: origin unknown. Conjectures have been offered as to its being a corruption of ‘adept’ or of ‘dapper’ but without any other evidence than appears in the general likeness and use of the words. It is possible that it is a derivative of DAB v. - ‘To strike somewhat sharply and abruptly.’” With the OED unable to shed any more light on the issue, the jury is still out as to the exact origin of the phrase.