Some readers (and editors, too!) nearly pass out when they see writers mix up these two words.
Faint generally means to deficient or barely perceptible, as in a faint light or to lose consciousness (e.g. Surprisingly, very few people ever faint during an earthquake in Los Angeles.).
Feint is to deceive through a mock action, as in The New York running back feinted going left but went right and scored.
Now that you know the difference, there’s no reason for you to ever again find these two words tricky!