A sacrifice performed with no regard for the prescribed rules, with no food given out, without sacred chants, with no gifts to the priests, and empty of faith, is called tamasic.
Explanation
Krishna is finishing his description of three kinds of sacrifice (yajña, ritual offering), each tied to one of the three qualities of nature (guṇa). After the sattvic and rajasic kinds, he describes the tamasic kind, marked by darkness and dullness (tamas). It ignores the rules, gives no food or gifts, omits the mantras, and lacks faith (śraddhā).