A wild animal's lair or resting place; a small, cozy room used for relaxation or work; a place where people gather for illicit activities.
From Old English 'denn' meaning 'valley' or 'hollow,' related to 'denu' (valley). The sense evolved from natural hollows to animal homes to human spaces, maintaining the idea of a sheltered retreat.
The evolution from 'valley' to 'animal lair' to 'cozy room' shows how 'den' has always been about enclosed, sheltered spaces. Even a 'den of thieves' maintains this core meaning of a hidden, protected gathering place.
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