den (n.)
Old English denn "wild animal's lair," from Proto-Germanic *danjan (cognates: Middle Low German denne "lowland, wooded vale, den," Old English denu "valley," Old Frisian dene "down," Old High German tenni, German tenne "threshing floor," from PIE *dan- "low ground"). Sense of "small room" is 1771, originally colloquial.