(In). Vague; a method of speaking or writing which always leaves a way of escape. The allusion is to the custom at fairs, etc., of soaping the tail of a pig before turning it out to be caught by the tail.
“He is vague as may be; writing in what is called the `soaped-pig'
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894