Bhagavad Gita Chapter 18, Verse 39

यत्तदग्रे चानुभवे च दुःखमेव सर्वतः | मोहादात्मविपर्यासात्तत्सुखं तामसं स्मृतम् ||

yat tad agre ca anubhave ca duḥkham eva sarvataḥ mohād ātmaviparyāsāt tat sukhaṁ tāmasaṁ smṛtam

That happiness which is suffering in the beginning and in experience, in all ways, from delusion and perversion of the self, is remembered as tamasic.

Krishna explains tamasic happiness. 'Yat tad agre ca anubhave ca duḥkham eva sarvataḥ'—that happiness which is suffering (duḥkham eva) in the beginning (agre) and in experience (anubhave), in all ways (sarvataḥ). 'Mohād ātmaviparyāsāt tat sukhaṁ tāmasaṁ smṛtam'—from delusion (mohāt) and perversion of the self (ātma-viparyāsāt), that happiness (tat sukham) is remembered (smṛtam) as tamasic (tāmasam). This is tamasic happiness: completely deluded, actually suffering but mistaken as happiness. It's suffering from the beginning, in experience, and in all ways, arising from delusion and perversion of the self. This is the lowest kind of happiness: not actually happiness at all, but suffering mistaken as happiness. Unlike sattvic happiness (difficult initially but nectar-like in the end) and rajasic happiness (pleasurable initially but poison-like in the end), tamasic happiness is suffering from the start, completely deluded. This is the path: recognizing deluded happiness and moving toward true happiness.