Green’s Dictionary of Slang

two shakes of a lamb’s tail phr.

also fling of a cow’s tail, half a shake of a lamb’s tail, half a shake of a dog’s tail, jerk of a lamb’s tail, three shakes, shake of a lamb’s tail, three shakes of a (dead) lamb’s tail, …of a (dead) sheep’s tail, two flips of a humming-bird’s tail, two shakes of a dead lamb’s tail, …of a (dead) sheep’s tail, …of a duck’s tail, …of a frog’s whisker, …of a gnat’s chuff, …of a lamb’s arse, …of the mainbrace, two jerks of a lamb’s tail, two twists…, two winks of a mole’s eye, whisk of a lamb’s tail

[mid-19C+] a very short time, usu. with in a..., meaning extremely quickly, at once.