to freak out, to crack under stress or pressure.
Specialized vocabulary used by scientists and engineers.
"You'd better stop interrupting, man. Dave's about to go nonlinear!"
Not simple, not easy to resolve. Comes from math: whenever you see the word "linear", you know it's easy.
This word is not restricted to use in math.
y'' = -sin(y) <- The nonlinear pendulum.
y'' = -y <- Linearized (nonrealistic) pendulum.
"meselfs, I have girl troubles. I have no friends so I'm asking you, a known dateless loser."
"Sounds pretty nonlinear. Unfortunately, I can only suggest you go tensile test yourself."