Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.4.4

3.4.4.4

tadvai devā aspṛṇvata | ta etābhirupasadbhirupāsīdaṃstadyadupāsīdaṃstasmādupasado nāma te puraḥ prābhindannimāṃlokānprājayaṃstasmādāhurupasadā puraṃ jayantīti yadahopāsate tenemām mānuṣīm puraṃ jayanti

The gods then prevailed. They besieged them by these sieges (upasad); and because they besieged (upa-sad) them, therefore the name Upasads. They clove the castles and conquered these worlds. Hence they say, 'A castle is conquered by siege;' for it is indeed by beleaguering that one of these human castles is taken.