Weasels suck eggs. Hence Shakespeare-
The weazel Scot
Comes sneaking, and so sucks the princoly egg.
Henry V., i. 2.
“I can suck melancholy out of a song, as a weazel sucks
eggs.” —As You Like It, ii.
To catch a weasel asleep. To expect to
find a very vigilant person nodding, off his guard; to suppose that
one who has his weather-eye open cannot see what is passing before
him. The French say, Croir avoir trouvé la pie au
nid (To expect to find the pie on its nest). The vigilant
habits of these animals explain the allusions.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894