Meaning of 'in two shakes of a lamb's tail'

[Pavell]

Test No. incompl/advan-71 "Two For You", question 4

I promise I'll be very quick indeed and I'll be back in two ......... of a lamb's tail.

(a) moves
(b) shakes
(c) twitches
(d) wags

Test No. incompl/advan-71 "Two For You", answer 4

I promise I'll be very quick indeed and I'll be back in two shakes of a lamb's tail.

Correct answer: (b) shakes

Your answer was: incorrect
I promise I'll be very quick indeed and I'll be back in two wags of a lamb's tail.
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Hi!
Please explaine way not "wags"
Thanks.

[Torsten]

Because in two shakes of a lamb's tail is an idiom which means in a very short period of time/very fast/in no time.
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