Generally, just means 'before' but is much easier, and funner to say. Commonly said by people who are drunk and when talking about the past slur and distort 'before' untill it just comes out as 'befur'.
Alvin: How many kids have you had?
Jessica: 7, i told you that 2 miniuted ago.
Alvin: LIAR! You never had any children befur!