The works of Ādi Śaṅkarācārya

Sanatsujātīyam

p. 268

p. 268

सर्वार्थानां व्याकरणाद्वैयाकरण उच्यते ।

In plain words

Because he takes apart the meaning of every word and lays it open, he is called a grammarian.

Close to the Sanskrit

From the analysis of all meanings, he is called a grammarian.