As a verb, phase means to plan or carry out systematically. It’s usually followed by in or out. For example, when you implement a plan little by little, you phase it in. When you abandon a plan little by little, you phase it out. Faze means to disrupt the composure of. If you are not bothered by something, you are unfazed.
Faze derives from the now-obsolete feeze,1 which had several meanings, including (1) to drive away, and (2) to frighten.2 The word developed from this source in the American West during the 19th century.3 In early use, it was sometimes spelled phase, but the modern spelling was standard by around 1900.
Phase is older, having come to English from French in the 17th century.4 It originally referred primarily to phases of the moon and other celestial objects. The newer, nonastronomical senses developed in the 19th century.3
The words are easily mixed up. As with many homophone pairs, the more common one (phase) is often used in place of the less common one (faze)—for example:
Losing Randy Moss didn’t phase him. [Opposing Views]
So the fact that there are still garden chores to do does not phase me. [Baltimore Sun]
These writers spell the words correctly:
Being told to “screw off” didn’t faze Mayor John Williams during his inaugural speech before a crowd of more than 200 people Monday. [Trentonian]
On Monday, the DOE released a list of 11 schools set to be phased out and a charter school recommended for non-renewal. [NY1]
Uncanny replicas faze monkeys, too [Futurity]
China will phase in planned changes to its loan-loss provisioning rules to give banks time to adapt. [Reuters]
Totally unfazed by the elements, he threw for 369 more yards and two more touchdowns. [Boston Globe]