"fragrant plant of the mint family," c. 1300, from Anglo-French lavendre, Old French lavendre "the lavender plant," from Medieval Latin lavendula "lavender" (10c.), perhaps from Latin lividus "bluish, livid" (see livid). If so, it probably was associated with French lavande, Italian lavanda "a washing" (from Latin lavare "to wash;" from PIE root *leue- "to wash") because it was used to scent washed fabrics and as a bath perfume.
The adjective meaning "of a pale purple color, of the color of lavender flowers" is from 1840; as a noun in the color sense from 1882. An identical Middle English word meant "laundress, washerwoman;" also, apparently, "prostitute, whore; camp follower" and is attested as a surname from early 13c.