Third-person singular present tense of 'faze,' meaning to disturb, disconcert, or confuse someone.
From 'faze,' an American dialect word dating to the 1800s with uncertain origins, possibly from Middle English or dialectal 'feaze' meaning to twist or worry.
The 'not fazed' expression became so common in the 20th century that many people use it without knowing faze is a relatively recent American addition to English!
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