This is an American colloquialism from the early years of the 1900s.
The Daytona Beach Morning Journal reported on July 27, 1973 that then-Ambassador Daniel P. Moynihan sent a cable to the U.S. State Department threatening to resign. In part, Mr. Moynihan wrote:
I quite understand that it might appear that we are off our rocker out here, but it comes down to a simple matter of good faith. The trust account agreement of May 1966 … states that nonexpendable property shall be transferred to the Government of India when no longer required for the support of the U.S. assistance program. We might have tried to weasel out, but you will need another Ambassador for such work. The U.S. keeps its word.
Nearly 20 years earlier, on May 19, 1955, The Spokesman Review out of Spokane, WA headlined an article reporting on the dissent within President Eisenhower’s administration with regards to minimum wage proposals. The headline boldly proclaimed: “Extending Wage Floor Is Urged: Solon sees Effort to ‘Weasel Out’ of Proposal.”
A decade before that the St. Petersburg Times reported on February 13, 1944 that:
There is no tendency here to even think about any peace that would let Japan weasel out of complete occupation by Allied troops.
The Los Angeles Times published an article on February 16, 1938 entitled “Farmers of Pacific Coast Organize to Defend Rights” where the staff reporter wrote:
Rathburne said the board had tried to “weasel out of a tight situation and to play politics” including Postal employees in the Northwest and in Boston, Kansas City, Indianapolis and Jacksonville from the election.
While there appears to be good reason to believe that the phrase was used in the mid-1920s, since it was used with ease by print journalists in the 30s with the expectation that readers would know what was meant by the phrase . Still, the earliest published use of the phrase “weasel out” is from 1938.
Historically Speaking 