armchair general
One who speaks authoritatively on topics one actually knows little to nothing about. My uncle is such an armchair general about the classes I'm taking—the fact that he never went to college doesn't stop him from weighing in! Stop being an armchair general and let me fix my car, since I actually know what I'm doing here!
armchair general
A self-proclaimed military expert with little or no practical experience, who imposes his or her views on others. See also backseat driver; Monday-morning quarterback.