If "nectar" refers to the sweet liquid produced by flowers:
* Pollen: While not technically "opposite," pollen is the powdery substance that flowers produce alongside nectar.
* Bitter: Describes a taste opposite to sweet nectar.
* Acrid: A strong, unpleasant taste, the opposite of the sweetness of nectar.
If "nectar" refers to something extremely pleasant or delightful:
* Poison: A substance that can cause harm, the opposite of something delightful.
* Disgusting: Describes something extremely unpleasant, the opposite of something enjoyable.
* Vile: Describes something morally reprehensible, the opposite of something pure and delightful.
Ultimately, the best opposite word for "nectar" depends on the specific context of your sentence.