hiding in the nest; staying hidden in one's nest
Refers to an animal or person staying concealed inside a nest or den; often implies avoiding the outside world.
The bird is hiding in its nest, protecting its chicks.
He stayed holed up in his room all day reading a book.
Compound of 巣 (su, 'nest') and 隠れ (kakure, 'hiding'), from the verb 隠れる (kakureru). The exact historical derivation is uncertain, but the meaning is transparent from its components.