Infantile Pillock 

A child-like or child-sized penis.

Infantile: relating to infancy
Pillock: a fool or idiot; a penis

Person1: You're a jerk.
Person2: Butthead.
Person1: Infantile Pillock.
Person2: ... did you just call me a small dick? YOU'RE the infantile pillock.
Person1: Haha.
Person2: What's funny is that you aren't disagreeing...

Infantile Pillock by princess.peach December 30, 2009

Infantile Pillock 

A childish and stupid person.

Dude A: "Why are you laughing at that stupid joke someone just told you?"
Dude B " I really can't help it."
Dude A " You really are acting like an infantile pillock."

Infantile Pillock by Hay, Dan Newman, Dan the Man July 20, 2016