Not to be successful in doing something.
To become weaker and less likely to recover.
In education, to have a level of performance too low to be acceptable as evidence of learning, whether on a single test or piece of work or for a whole course; in the latter case a teacher would not allow a student credit for taking a course. Sometimes the failing level is a preset grade, such as below a D average or a numerical average below 65 or 70; but sometimes the “fail” label simply distinguishes those who did not receive credit from those who did, in a pass-fail system.