I've always been a little vague about what "gaslighting" means.  Therefore, I went to my good friend, Wikipedia, and here's the definition: 

Gaslighting - Wikipedia
https://en.wikipedia.org/wiki/Gaslighting
Gaslighting or gas-lighting is a form of manipulation through persistent denial, misdirection, contradiction, and lying in an attempt to destabilize and delegitimize a target. Its intent is to sow seeds of doubt in the targets, hoping to make them question their own memory, perception, and sanity.

If you follow Mr (soon to be President) Trump's comments and tweets, you will understand Mr. Rosemeyer's use of the term in reference to him.  From a "reality "TV" star it was expected, from candidate Trump it was, first, amusing, then increasingly concerning; from the soon-to-be President, it's simply frightening.  Yes, that's my OPINION.  If you think his ever-changing tweets and denial of reality are OK, please explain why.....it might make me feel better.  Thanks, Gary 
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