Imbecilic (adjective)

1. Extremely stupid; showing a lack of intelligence or common sense.
2. Having or showing a mental disability; having or showing characteristics of an imbecile.

Origin:

Early 19th century: from french imbécile, from latin imbecillus "feeble, weak", from in- "not" + roborare "strengthen".

Examples:

1. It was imbecilic of him to think he could drive home after drinking so much.
2. His imbecilic behavior caused him to lose his job.
3. The plan was so imbecilic that it was bound to fail.
4. He was called imbecilic by his colleagues for his lack of knowledge on the topic.
5. It was imbecilic of her to go outside in such bad weather without a coat.