we use it to threat someone . ويل is river of fire in the hell

The word ويل means woe, torment, misery, evil, disaster. So يا ويلك is a warning word of a bad or very bad consequence of an action.

It used for joking such as " i will kill you " or " i will punish you later " or " wait for your punishment "

Edited.....commented bellow

Some times we can use it for joking