“That’s not recommended.” 

Engineering speak for “absolutely fucking do not do that.” Usually used to answer a question about an operating state that is obviously destructive.

Sometimes can be used less severely to describe exceeding safety limits of a system.

Customer: “What if our reactor is overheating and instead of decreasing load we accidentally decrease coolant?”

Engineer: “That’s not recommended.”

Customer: “Our load has temporarily increased. We want to increase to 106% design capacity. Can the machine handle it?”
Engineer: “Possibly, but that’s not recommend.”

“That’s not recommended.” by Telltalehart July 26, 2021

That’s not recommended 

Engineer speak for “Do not fucking do that.”

Frequently in response to a phenomenally dumb question (usually by sales) or an absurdly improbable situation.

Meant to be derisive.

Sales: “What if we put the square peg in the round hole?”
Engineer: “That’s not recommended.”

Customer: “What if the lawn mower is still running when I stick my hand in?”
Engineer: “That’s not recommended.”

That’s not recommended by Telltalehart August 5, 2021