Herbert

1. A person (usually male) that is highly suspected of intending to cause mischief, damage, theft or unwanted sexual advances. Typically secretive in nature, they may be seen loitering for far too long in one area, waiting for an opportune moment to carry out said tom foolery.

Their intent can also be detected on their way to aforementioned misconduct.

2. Also known as 'Erberts' or 'Sherberts' (derived from 'Sherbet').

"Don't get involved with him; he's a right Herbert."

"Look at these two Herberts, they look like they're up to no good.

I'm calling the police."

"You take my word for it; He's a fucking absolute horrible little Herbert he is."