denyoudaoicibaDictYouDict[den 词源字典]
den: [OE] Related forms such as German tenne ‘threshing floor’ and possibly Greek thenar ‘palm of the hand’ suggest that the underlying meaning of den may be ‘flat area’. Old English denn denoted ‘wild animal’s lair’, perhaps with reference to animals’ flattening an area of vegetation to form a sleeping place. Dean [OE], a word for ‘valley’ now surviving only in placenames, comes from the same source.
[den etymology, den origin, 英语词源]
den (n.)youdaoicibaDictYouDict
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.