KanjiJLPT N1
Ghostly

seclude, confine to a room, deep, profound, secluded, faint, dark, tranquil, calm

Mnemonic

Two cocoons dangle from the ceiling of a dark mountain cave. One cocoon is white, the other gray—both empty, their inhabitants long gone. They sway in an unseen draft, casting ghostly shadows that seem almost alive on the cold stone walls.
Additional thoughts
Picture the two cocoons (幺幺) nestled beneath the mountain (山) cap of the kanji. The emptiness of abandoned cocoons in a deep cave perfectly evokes the dim, otherworldly feeling of ghostly.
Quick recall
Two hollow cocoons swaying inside a mountain cave cast ghostly shadows.

Details

The keyword for 幽 is ghostly. This kanji conveys a sense of something dim, faint, secluded, or belonging to the spirit world. It suggests an atmosphere of deep mystery, darkness, and otherworldly quiet, as one might associate with phantoms or hidden, shadowy realms. By extension, it can also describe something subtle, profound, or elegantly subdued, carrying an air of the unseen and the ethereal.
On'yomi
ゆう
Kun'yomi
ふか.い、かす.か、くら.い、しろ.い