俱舍論記 4.115

4.115

「於未作事至是我惡作」者，釋。於未作事業亦立作名。如追悔言：我先不作如是受戒等事業，是我惡作。此即緣未作事亦名惡作。亦得從境為名。

Regarding "With respect to unperformed matters [...] 'that is my bad action,'": this is the explanation. The designation "action" is also applied to unperformed undertakings. As in the subsequent regret: "Formerly I did not perform such an undertaking as receiving the precepts, and that is my bad action." This means that taking an unperformed matter as an object is also called bad action. It may also derive its name from the objective sphere.