Incompetent means not able to do the job properly. The plumber who floods the kitchen, the manager who loses the same file twice, the driver who cannot reverse into a space.
It is competent with in- on the front, and it stings: calling a colleague incompetent is an insult, not a neutral report. The noun for the quality is incompetence, and the word doubles as a noun for the person — a pack of incompetents. In law it turns narrow and cold: someone judged incompetent cannot manage their own affairs or stand trial. Keep it for people and their work. A kettle that has stopped boiling is faulty, not incompetent.
1adjectiveincompetentLacking the skill or ability to do something properly.
“The report blamed an incompetent supervisor for the delay.”
2adjectivejudgedJudged in law to be unfit to manage one's own affairs, give evidence, or stand trial.
“The court found him incompetent to stand trial.”
3nouncannotA person who cannot do their job properly.
“The whole department is run by incompetents.”
Incompetent reached English through French incompétent, from Latin incompetēns. Pull it apart and it is nothing more than competent with in- stuck on the front — the same 'not' that gives incomplete and incorrect. The whole insult is built out of a prefix.
Latin
incompetentem
French
incompétent
English, today
incompetent
French incompétent, from Latin incompetentem, from Latin incompetēns, equivalent to in- + competent.
Etymology from Wiktionary, CC BY-SA