अदेशकाले यद्दानमपात्रेभ्यश्च दीयते। असत्कृतमवज्ञातं तत्तामसमुदाहृतम् ॥

adeśha-kāle yad dānam apātrebhyaśh cha dīyate asat-kṛitam avajñātaṁ tat tāmasam udāhṛitam

That alms given at unfit place and time, and to unworthy persons, disrespectfully and contemptuously, that is declared of darkness.

In plain words: A gift is dark when it is given at the wrong time or place, to the wrong person, or with rudeness and contempt.