Vaisesika Sūtra 6.2.06

अशुचीति शुचिप्रतिषेधः ॥६॥

aśucīti śucipratiṣedhaḥ ॥6॥

[अशुचि (aśuci) - impure; इति (iti) - thus; शुचि (śuci) - pure; प्रतिषेधः (pratiṣedhaḥ) - negation / opposite of;]

Impure is the opposite of purity.