The phrase "a complete nut" is correct and usable in written English.
It is typically used to describe someone who is eccentric, crazy, or behaves in a wildly irrational manner.
Example: "After hearing his wild conspiracy theories, I realized he was a complete nut."
Alternatives: "a total lunatic" or "a real weirdo".
"I'm a complete nut about sports," he continued.
At the time I thought, This man has got to be a complete nut.
"You almost have to be a complete nut, if you want to say it that way," Tyree said.
"I guess I can say I'm a complete nut, because I'm not the biggest guy, but I'll do what it takes to get the job done".
"It's sad the choices we have in the elections this year: one that's a complete nut and the other who is dishonest," said Lee, a 68-year-old retiree, referring to Republican front-runner Donald J. Trump and Clinton.
"I'm not sitting here saying I have a lot of problems or I'm a complete nut-job," he said last week.
Then Philippe Henry, the owner and a complete wine nut, overheard us discussing Bordeaux (where my friends work) and figured he'd found some kindred spirits.
Mail Order Demeter 12 Welles Street, Sandbach, Cheshire CW11 1GT, 01270 760445 Run by a complete ginger nut, Demeter has a full range of ginger-themed products, including ginger chewing gum.
I'm a complete gadget nut and I love the wireless charging, but you can buy an awful lot of Lightning cables and much more powerful battery packs for the same price.
His own father was "a complete Western nut," a buff who frequently would tear up while watching "The Outlaw Josey Wales". Eventually, Morgan started tearing up at "Josey Wales" too, but the rocky road to Western art wasn't direct.
In this case, it is difficult to attribute nut loss to birds or non-bird factors that may result in complete nut loss (e.g. storm damage).
