From Sefer Ha-Hayim:
What is migo? It is a common talmudic legal device that gives litigants believability when they otherwise would not have it. Since - migo - he could have lied and claimed X and been in a better position, the fact that he did not claim X means that he must be telling the truth. This never sat well with me. In a footnote to R. Michael J. Broyde's The Pursuit of Justice and Jewish Law, the author explains in contemporary legal terms how a migo works... (continued here)