linden n.

Old English liðe "soft, mild, gentle, calm, meek," also, of persons, "gracious, kind, agreeable," from Proto-Germanic *linthja- (source also of Old Saxon lithi "soft, mild, gentle," Old High German lindi, German lind, Old Norse linr "soft to the touch, gentle, mild, agreeable," with characteristic loss of "n" before "th" in English), from PIE root *lento- "flexible" (source also of Latin lentus "flexible, pliant, slow," Sanskrit lithi).

In Middle English, used of the weather. Current sense of "easily flexible" is from c. 1300. Related: Litheness. Old and Middle English had the related verb lin "to cease doing (something)," also used of the wind dying down.

European tree of the poplar family, late 14c., from adjectival or genitive form of Old English æspe "aspen tree, white poplar," from Proto-Germanic *aspo, adjective aspin- (source also of Old Norse æsp, Middle Dutch espe, Old High German aspa, German Espe), from PIE *aps- "aspen" (source also of Lithuanian epu, Latvian apsa, Old Prussian abse, Russian osina), perhaps a northern European substratum word.

The current form in English probably arose from phrases such as aspen leaf, aspen bark (see -en (2)). Its leaves have been figurative of tremulousness and quaking at least since early 15c. (an Old English name for it was cwicbeam, literally "quick-tree").