Definition
A sarcastic remark indicating that the speaker is not scared.
How People Actually Use It
Nowadays used in a passive aggressive way (usually by the british) at school or somewhere public when someone tries to act big and bad or naughty when they're really just a try hard. For example:
"Person 1: Talk shit about me one time and I'll batter you - you hear me? Person 2: Ooh, sorry roadman - I'm so scared - shiver me timbers!"
Source: Urban Dictionary