Designating a gauge, or part of a gauge, that will not pass through an aperture of the required size, or will not admit an object of the required size.
Origin
Early 20th century. From not + go.
Definition of not-go in US English:
not-go
adjectiveˈnɒtɡəʊ
Engineering
Designating a gauge, or part of a gauge, that will not pass through an aperture of the required size, or will not admit an object of the required size.