Rubber faced sidekick. A few cards shy of a full deck. Doesn't have both oars in the water. Funny as a screen door in a submarine. You make as much sense as a pay toilet in a diarrhea ward.
I was at a Birthday Party and my 70yo Mother was showing a younger girl how to use a digital camera.
The young girl said, "if you can do it, so can I" / right away I noticed the (unintended) insult.
Back when I was doing tech support, I made that comment on internal correspondence when one of the developers asked if the user had a clue on what he was doing. I heard the people in QA got a kick out of that comment along with the developer.