head space and timing 

This is a military term related to ensuring the bolt and head space for the M2 .50 caliber machine gun are in alignment. This must be done correctly, or the weapon can explode during operation. It also means a person who's screwed something up because they failed to check what they were doing before they did it.

Lieutenant Smedlap blamed his mistakes on his platoon sergeant, but it was actually his fault.

"LT Smedlap blamed his sergeant, but it was really a head space and timing problem"

head space and timing by Soldier online October 23, 2009

head on a swivel 

To be constantly moving your head left and right so that you do not miss anything approaching you from a blind spot

Mandy had her head on a swivel as she rode her bicycle in downtown traffic in order to avoid getting hit by a car.

head on a swivel by Chow-bi July 5, 2017

head world 

A head world is generally the imaginary world of an artist or writer in which their characters interact and their experiences take place. Head worlds usually have some sort of theme or their own settings and originality, whether in characters, or environment dynamics.

Finn and Jake are characters in the "OOO" head world in the show "Adventure Time".

Harry, Ron, and Hermione are three very important characters in J.K. Rowling's head world.

"Wow, I am just itching to write more about my characters and their experiences; I love writing about my head world."

head world by RainbowFoxy February 24, 2015

Head Canon 

An idea or opinion about a fictional series (Book, TV, Comic, or otherwise) that is true in one's head, but has not become a canon fact.

In my head canon, Jade and Dave from Homestuck are totally in love with each other.

Head Canon by Nellaxival August 4, 2011

head nest 

a scarf wound somewhat tightly around one's neck, thus creating a head nest

Rainy, cold days are the most opportune times to wear a head nest.

head nest by dystopianchinscratcher October 31, 2013

head bussa

Someone who can fight; To bust a head
Usually usedd down south.

Terry always beatin speople up, he a head bussa.

head bussa by ShAdY September 6, 2003

Head pigeons 

A serious, serious, illness involving having pigeons on one's head. The cure has proven to be elusive and remains highly contagious.

Ms. Bitters: There is a pigeon on your head. You've got Head pigeons. Get to the nurse before they spread to the other children!
Zim: Yes, Ms. Bitters.

Head pigeons by Salad Sister December 16, 2009