Oh, I actually quite like the suggestion of 'spy'? The Prince is annoyed at Ewen butting in where he isn't wanted and asking questions, and there aren't that many short words beginning with 's' that would express that.

I did wonder if the word might be 'scrub', which seems to be a sort of all-purpose insult for someone stupid and useless, but that doesn't really fit the scene, since Ewen is actually being ultra competent - too competent, in the Prince's view. :)