Entry from OED Online

[Shortened < *porky pie s.v. PORKY a.] 

    A lie. Usu. in pl.
 
  1985 J. SULLIVAN Only Fools & Horses (1999) I. 4th Ser. Episode 2. 217 Rodney. You don't believe all them stories do you? Del. What? Do you reckon they're porkies? 1989 Financial Rev. (Austral.) 7 Apr. 14/2 A code of silence operates among players... They feign amnesia. They prevaricate. They tell porkies. 1992 Folk Roots Sept. 54/1 It's the same defensive posture taken by people who start sentences with the words, ‘to be honest with you’; instant confirmation that they are about to tell a whopping great porky. 1995 FHM Sept. 94/1 No one in their right mind ever seduces a woman without having recourse to distortion, lily-gilding and downright porkies. 2001 Mirror (Electronic ed.) 21 Mar., Scientists found that more blood is pumped into our nasal tissues when we tell porkies—causing them to expand.