याज्ञवल्क्येति होवाच शाकल्यः ।यदिदं कुरुपञ्चालानां ब्राह्मनानत्यवादीः किं ब्रह्म विद्वानिति ।दिशो वेद सदेवाः सप्रतिष्ठा इति ।यद्दिशो वेत्थ सदेवाः सप्रतिष्ठाः ॥३,९.१९ ॥
In plain words
"Yājñavalkya," said Śākalya, "knowing what Brahman is it that thou hast overcome these brahmins of the Kurus and Pañcālas?" "I know the quarters with their deities and their supports." "If thou knowest the quarters with their deities and their supports —
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
ष्ठातृभिः, किंच सप्रतिष्ठाः प्रतिष्ठाभिश्च सह । इतर आह— यत् यदि दिशो वेत्थ सदेवाः सप्रतिष्ठा इति, सफलं यदि विज्ञानं त्वया प्रतिज्ञातम् ॥
In plain words
...by their supports. Moreover, he means the directions come with their own foundations too. Shakalya answers: if you really do know the directions as things standing together with their gods and their supports, then this knowledge you claimed to have is finally worth something.
Close to the Sanskrit
'...by those who stand as support' — moreover, [the directions are] 'accompanied by supports', that is, together with their supports. The other one [Śākalya] says: if indeed you know the directions as accompanied by their deities and their supports, then the knowledge you have professed is fruitful.