In poems, it often has erotic and religious undertones. But I've also wondered if it is associated with death as well as sleep; if it is in flower or is used in perfume form in wakes and funerals. The context is of a sexy, morbid, funereal splendor and perhaps of a heavy overpowering incense. In fact I think it might be a common flavor of incense. It's the sort of thing that a hero of an H. Rider Haggard novel would be drugged by as he goes into Ayesha's secret temple. García Lorca seemed fond of it; it's all over his poems.
The US English-speaking reader isn't going to have the faintest clue what nard is. Here is a perfect place to choose domesticizing over foreignization. I've translated it differently in different poems. "Scented lily?" "Costly balm?" I've translated it as "myrrh" but am not entirely happy with that. "Fragrant valerian?" "Holy patchouli"? (That's a joke.)
For example, here in "Insomnia" by Juana de Ibarbourou, we run into some nard:
En un zumo de lirios morados
Se anegan mis ojos sombríos y largos
Y en un zumo amarillo de cera
O de vara de nardo marchita,
Se han ahogado las llamas rosadas
Que coloran la piel de mis labios.
What are those varas de nardo? I thought they could be withered stalks of the flower, but also they might be crumbling sticks of incense. Maybe it is obvious to a Catholic Spanish-American. If so, I'd love to know!
My point is really that I've seen a lot of translators interpret it by a dictionary definition without thinking about it or digging deeper, and without trying to make the meaning of the flower/unguent/incense apparent to their readers.
Untranslated and unexplained, I think "nard" in a poem makes that poem fall flat in English.