A preferred bathroom of choice in a familiar environment (usually workplace) which is either further away than the designated bathroom intended to be used or off-limits for any reason such as gender. Often the journey to any lafferty involves clandestine tactics to avoid suspicions of those who have that bathroom rightfully designated to them.
When referring to a known majority preference amongst a group then it is referenced as "the lafferty"; otherwise, the possessive form "my lafferty" would be used.
Matt: Where are you off to so quickly?
Ian: Trying to sneak by the girl at the front desk so I can spend a few minutes in the lafferty. Do me a favor and explode something while I run by?