The term is not used today, but "lamb" was popular from the 1880s.
(Oxford English Dictionary)
A simpleton; one who is cheated; esp. one who speculates and loses his money.
1668 Leathermore's Adv. conc. Gaming (ed. 2) 5 When a young Gentleman or Prentice comes into this School of Vertue unskil'd in the quibbles and devices there practiced, they call him a Lamb. 1680 COTTON Compl. Gamester (ed. 2) 5 And then the Rooks..laugh and grin, saying the Lamb is bitten. 1881 J. MILLS Too fast to last III. x. 127 'In orderThat we may not be among the skinned lambs', interrupted William Bottles. 1884 Chicago Tribune Feb., 'Lamb' is an outsider who goes into the market and leaves his money. 1886 GLADDEN Applied Chr. 204 A recent estimate..puts the amount of which the 'lambs' are shorn in this New York stock market alone at eight hundred million dollars a year.
22 November 1879, Washington Post, pg. 1:
THE BULLS AND BEARS.
A TREMENDOUS CRASH IN THE WALL
Black Friday Redivivus -- A Tumble in Stocks Which
Sent Leading "Lambs" to the Wall -- The
Market Recovers from the Shock -- Jay
Gould's Tremendous Scheme.
As predicted yesterday, the downward boom began at the first call, and there was a lively shaking up of brokers who had been operating on small margins and havoc among the lambs who had been drawn into the vortex of speculation by the hope that the upward boom in prices would continue indefinitely.
29 July 1882, Literary World, pg. 256:
A BURLESQUE TRAGEDY,
BRIGGS, a broker (Briggs, Brown & Co.)
HOBBS, clerk of Briggs.
PHIPPS, a customer.
CULLY, janitor of Briggs.
MIKE, a telegraph boy.
Choruses of Bulls, Bears, Shorn Lambs, etc.