If you can't dazzle them with brilliance, baffle them with bullshit.

The earliest source found in Google Books dates from 1958, and does not attribute it to W. C. Fields but to a certain "Bill": I said, "What do you do, Bill?" He says "Well, if I can't dazzle them with my brilliance I baffle them with my bull," and doggone if he doesn't. Not found attributed to W. C. Fields until 2005.

Note, “Bill” is a common salutation for “William.”