Origins – Swing and Sway.

Vacillate – note the single c, double l – derives from Latin vacillo, swing back and forth.
People who swing back and forth in indecision, who are irresolute, who can, unfortunately, see both, or even three or four, sides of every question, and so have difficulty making up their mainds, are vacillatory. They are also, usually, ambivalent – they have conflicting and simultaneous emotions about the same person or thing; or they want to go but they also want to stay; or they love something, but they hate it too. The noun is ambivalence – from ambi both.
Ambivalence has best been defined as watching your mother-in-law drive over a cliff in your new car.
To vacillate is to swing mentally or emotionally. To sway back and forth physically is oscillate – again note the double l – (OS’-i-layt’), from Latin oscillum, a swing. A pendulum oscillates, the arm of a metronome oscillates, and people who’ve had much too much to drink oscillate when they try to walk.
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