"petty crook who will turn his hand to anything so long as it does not involve honest work," 1934, British slang, probably dating back to late 19c. and connected with spiff (see spiffy) in one of its various senses. Being a flashy dresser was a spiv characteristic.
The spiv reached his apotheosis during World War II and the succeeding years, when the disrupted economic conditions allowed ample scope for unofficial trading (a pair of nylons here, a few packets of cigarettes there) and other petty crime. He became a stock figure in the English social comedy, represented on screen by such stereotypes as 'Flash Harry' (played by George Cole) in the St. Trinian's films and Pte. Walker in Dad's Army. [Ayto, "20th Century Words"]