This is an expression to indicate whacking off (hence whack-off-ia). The bank part comes into the picture because every time you whack off, you usually deposit something.
John: Hey Paul, you spider monkey, have you been wanking all night again??
Paul: Hush John! Don't say it loudly. But yeah I made a large deposit at Whackoffia Bank last night.
Paul: Hush John! Don't say it loudly. But yeah I made a large deposit at Whackoffia Bank last night.
by SantoshGeorge February 17, 2008