I’m a practicing occultist, so my knowledge of the gesture is from that angle. I’m at work right now, so I can’t drag up the best sources at the moment, but here’s a Wikipedia article about it:

en.wikipedia.org/wiki/Fig_sign

And here is a Wikipedia article about hand gestures that mentions the “Clinton Thumb” under the single handed gestures section:

I believe I first heard the Masonic political connection from Lon Milo DuQuette, who I believe is a Mason as well as a practicing occultist like myself. He has over 1000 videos on occultism on YouTube, and I think I probably heard it from there as well as other sources about occult hand gestures.

For the record, I’m not saying it is an occult gesture to imply any conspiracy—just that the history of Masonry and other occult interests in politicians over history have led to certain themes and gestures blending in to the common political vernacular.