A mostly uncommon euphemism for a brain, more specifically the human brain. While the brain and nervous system is not necessarily circuitous in nature, it has been often referred to in the parlance of modern technology.
Meat circuit is a wonderful phrase that can add color to any conversation, as is demonstrated thusly:
"I was trying to explain Sendmail rulesets to Bob when
his meat circuit went into latch up."
"You earthlings with your puny meat circuits are no match
for the Squid People!"
The utility of this phrase in modern English
is unfortunately overlooked by most people. Please use it in your place of business