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1.
Someone whom has had sexual relations with a person whom you have also had sex with.
O me and lisa go way back we both were shagged by giles so we are fuck in laws too.
by Sirus May 15, 2003
 
2.
The relationship you have with anyone that has already had sex with the person you’re currently sleeping with.
Bob banged Susan at that party last year, guess that makes us fuck-in-laws.
by Sigar Cel November 25, 2003