A gift that is given at the wrong place and time, to unworthy recipients, without respect and with contempt, is declared to be tamasic.
Explanation
Krishna is finishing his three-fold account of giving (dāna), after the sattvic and rajasic kinds. Tamasic giving (tāmasa), the kind shaped by darkness and ignorance, is marked by bad timing and place, unsuitable recipients, and a lack of respect or even scorn toward the person receiving. Commentators differ on who counts as an "unworthy recipient": some stress moral unfitness, others stress that the gift would be misused or harmful.