Definition of flunking:
(v) :
(ambitransitive) Of a student, to fail a class; to not pass.
(v) :
(transitive) Of a teacher, to deny a student a passing grade.
(v) :
(by extension) To fail any challenge
(v) :
To 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.)
(v) :
(dated, informal) To shirk (a task or duty).