This is a wild frenzy caused by desire for an unattainable ideal. Edward Bulwer-Lytton described its sense in Godolphin in 1833: “The most common disease to genius is nympholepsy — the saddening for a spirit that the world knows not.” It can also refer to the passion or desire aroused in men by young girls, which is, unsurprisingly, in Lolita by Vladimir Nabokov. As a result, it’s often equated with paedophilia or what’s sometimes called the Lolita complex, though it’s strictly an unappeasable longing, not one that can be acted upon.
Nympholepsy began life in English in the late eighteenth century, with the idea behind it of a person in a frenzy from beholding those mythological spirits of nature which the ancients imagined as beautiful maidens living in rivers or woods. It’s from the Greek numpholeptos, caught by nymphs. George Moore wrote about it in his Memoirs of My Dead Life:
I have always thought it must be a wonderful thing to believe in the dryad. Do you know that men wandering in the woods sometimes used to catch sight of a white breast between the leaves, and henceforth they could love no mortal woman? The beautiful name of their malady was nympholepsy. A disease that every one would like to catch.
By the early nineteenth century it had added the meaning in the definition, the one Lord Byron called “The nympholepsy of some fond despair”.