俱舍論疏 15.137

15.137

論「非前二業至善不善故」，釋無記業。安故名善，不安故名不善，非二故名無記，不可記為安、不安故。

The Śāstra says: "Karma that is neither of the preceding two [...] good or unwholesome." This explains indeterminate karma. Because it is secure it is called good; because it is insecure it is called unwholesome; because it is neither, it is called indeterminate, since it cannot be declared as either secure or insecure.