If your best friend calls you an imbecile, he's implying that you're stupid, and he's probably pretty angry with you. An imbecile is an extremely stupid person.
The noun imbecile is used informally as an insult to mean "fool". Its origins are in the Latin word imbecille, "weak or feeble," and it was an official medical term for people with a specific (and low) I.Q. in the 19th and early 20th centuries. Patients who were classified as imbeciles were said to have no more intelligence than a seven year-old child.