A phrase often used to preface an opinion on a subject when the person voicing the opinion is not an expert in the field under discussion.

The "I'm not a rocket scientist, but ....." when uttered by one's boss, the administration, or some other "concerned group" are the type of comments that leads a committee to design a camel when given the brief for a horse.

One of my professors (in Astronomy) was at a university senate meeting and recounts the following story. Someone began by saying "I'm not a rocket scientist, but .. ". The next person continued "I'm not a rocket scientist either, but ...". This continued around the table, until my professor could contain himself no longer.
Up he jumped and declared "Speaking as the only rocket scientist in the room ..." (he was working with an x-ray satellite at the time !)