Psycho

I know teaching reception children can be exhausting and somewhat frustrating due to their waning attention span and abundant energy.
However I was surprised to discover that my son's teacher had nicked named a couple of the more active boys the "Psycho Kids". The boys seemed to think this rather amusing.
The teacher luckily overheard their comments to me and through much laughter explained that she had actually called them her "side-kicks" as they had been very helpful the day before.
Now why is it they understood psycho more than side-kick?