Misnomer is used when names don't fit reality. Think of it as a misleading label. Calling him a professional was a misnomer; he lacked the necessary skills.
Pro Tip 2/3
Cultural References
Some misnomers persist because of history or tradition, even if they're outdated. Greenland is a misnomer given its icy landscape, but the name has stuck.
Pro Tip 3/3
Common Mistakes
Don't confuse 'misnomer' with a lie. It implies confusion or inaccuracy, not deceit. Saying he 'borrowed' it without asking was a misnomer; he actually stole it.