I am familiar with "two shakes of a lambs tail" but not the dead lamb variation. Here's a reference to "two shakes .."
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in two shakes of a lamb's tail
- very quickly and without difficulty
"Wait one minute. I will be able to help you in two shakes of a lamb's tail.http://"http://www.geocities.com/Athens/Aegean/6720/Number.html
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Your grandmother was not alone in her "dead lamb" variant. Here's a reference:
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QUICKER THAN TWO SHAKES OF A DEAD LAMB'S TAIL. Quick.http://www.napanet.net/~garate/Phmstd.htm
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It sounds like you were lucky that she stuck with the dead lambs. The next on the list is:-
ENOUGH TO GAG A MAGGOT OFF A GUT WAGON - yuk!