Definiciones
1.VerbOf a student, to fail a class; to not pass."He flunked math, again."
2.VerbOf a teacher, to deny a student a passing grade."Unsatisfied with Fred's progress, the teacher flunked him."
3.VerbTo fail any challenge
4.VerbTo shirk (a task or duty).
5.VerbTo back out through fear. (Commonly in the phrase 'flunk it', the 'it' referring to a specific task avoided; sometimes without specific reference, describing a person's attitude to life in general.)