What is the definition of Mostly Fear?

"Mostly fear is like a dark comedian who always walks in, dressed in a trench coat that says 'I'm here to make you uncomfortable', but secretly has a better sense of humor and only wants to scare you into buying more popcorn."

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Now, here's an edgy example sentence using 'ostly fear': "Despite being the most logical person in the room, John's heart was mostly fear when he heard the cryptic message on his phone."