俱舍論記 4.112

4.112

「惡所作體至說為不淨」者，答。惡所作之體名惡作。惡作是所緣境，體即正是追悔。又解：體之言事，惡所作事名為惡作。此即正解惡作。應知此中緣惡作法，心追悔性，說名惡作。此即從所緣立名。如緣空解脫門，體正是定，說名空者，從所緣立名。亦如不淨觀，以無貪為體，說為不淨，從所緣立名。

Regarding "The entity of what was badly done [...] is spoken of as impurity,": this is the answer. The entity of what was badly done is called bad action (kaukṛtya). The bad action is the objective support, and the entity itself is precisely subsequent regret. Another explanation: "Entity" means matter: the matter of what was badly done is called bad action. This directly explains regret. One should know that here, the mind's nature of subsequent regret which takes bad action as its object is called regret. This is naming it after its objective support. Just as the gate of liberation of emptiness has samādhi as its real entity, yet is named "emptiness" after its objective support; or just as the contemplation of impurity has non-attachment as its entity, yet is spoken of as "impurity" after its objective support.