serial sevens 

1. A psych test measuring cognitive ability where subject counts backward from a random number in increments of seven.
2. Any difficult mental calculation.

Explained in The Contortionist's Handbook by Craig Clevenger.

"I can figure out how much we all owe if you just let me see the bill... it isn't serial sevens."

serial sevens by Ben Frey April 5, 2006

Seven gilla serial killa 

a Tasmanian long line shark fisherman

Man that crew outta Dover has got some real seven gilla serial killas!

Seven gilla serial killa by Bad hombre May 24, 2022