The happiness that arises from the contact of the senses with their objects, that seems like nectar at first but like poison in the end, is called rajasic.
Explanation
Krishna is describing three kinds of happiness, matching the three qualities (guṇa) of nature. Here he describes passion-driven (rajasic, from rajas) happiness: the pleasure of the senses meeting what they crave, which feels wonderful at first but later brings exhaustion, craving or regret. It is the exact reverse of the sattvic happiness in the previous verse, which is bitter at first and sweet in the end.