"You're not a good fit."
This statement could mean a lot of things.
It could be innocuous description of a host of performance issues.
But it could also mean—
"You're not white."
"You're not male."
"You're not Christian enough."
"You're too Brown.
"You're too old."
"You're too disabled."
With this background, consider Lashley v. Spartanburg Methodist College, which involved a teacher suing her former employer after it did not renew her contact because they "were not a good fit for each other." The teacher claimed "good fit" was pretext for retaliation based on her prior request for a disability reasonable accommodation.
The 4th Circuit disagreed.
Describing an employee as not a "good fit" is an assessment that employers make all the time. Maybe someone's skills do not match up with the institution's mission. Maybe someone's work ethic falls short of expectations. Maybe someone is just not a good team player. Though there may be circumstances where evidence reveals that "good fit" is a subterfuge for discrimination or retaliation, it is also a perfectly innocuous comment that an organization's collaborative goals would not be furthered, and in fact might be retarded, by a particular employee. Institutional success is often a collective enterprise toward which an employer has entirely reasonable expectations that each employee should contribute.
While I understand the 4th Circuit's rationale in Lashley, I'm not comfortable with an employer relying on "fit" as justification for a personnel decision. That word is too vague, too loaded, and too open to an interpretation that could easily evade summary judgment and buy you a jury trial. Instead, document the employee's specific failings so that if you are called upon to explain why he or she isn't "a good fit," you have some non-discriminatory, non-retaliatory meat to put on that bone so that you're not leaving it in the uncertain hands of a judge or jury to interpret what you meant by "fit."