spare tiring

Third wheeling for two couples, so one would be the fifth person (fifth wheel), hence the name spare tire.

Ricky and Jane went with John and Rachel to the movies on a double date. They invited David too. David politely declined, however, telling them that he didn't want to spend a perfectly good Friday night spare tiring.

by Whitest Black Person October 27, 2016

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"

by Soldier online October 23, 2009