Śatapatha Brāhmaṇa ("of a hundred paths") 1.9.2.26

1.9.2.26

atha yasmātsamiṣṭayajurnāma | yā vā etena yajñena devatā hvayati yābhya eṣa yajñastāyate sarvā vai tattāḥ samiṣṭā bhavanti tadyattāsu sarvāsu samiṣṭāsvathaitajjuhoti tasmātsamiṣṭayajurnāma

Now as to why it is called samishtayagus: whatever deities he invites through this (new or full-moon) sacrifice, and for whichever deities this sacrifice is performed, all those are thereby 'sacrificed to together' (sam-ishta); and because he now makes a (butter) oblation to all those deities, who have been 'sacrificed to together,' therefore this (oblation) is called samishtayagus.