The word lavender has been around Middle and Modern English at least since its earliest recorded attestation in a book of Anglo-Saxon vocabulary, and as such has underwent a plethora of spelling variations through history. Alternate forms have included lavandre, lavendere, lavanda, and lavand, and it all comes from Old French lavandre, still with the same definition. That was borrowed something during the tenth century from Medieval Latin lavandula, which is thought to be from Latin lividus, meaning "bluish". The letter a was probably thrown in at the beginning because of confusion between that and the word lavare, meaning "wash" (because lavender was used in laundry in olden times - still is occasionally). Going further back, lividus is from the Old Latin verb slivere, "to be blue", and that's reconstructed as deriving from Proto-Indo-European sliwo, still referring to the hue. Usage of lavender over time peaked in the late 1920s but has been increasing again since the 1980s.
Hello! I'm Adam Aleksic, a sophomore studying linguistics and government at Harvard University, where I founded the Harvard Undergraduate Linguistics Society. I also have disturbing interests in politics, vexillology, geography, board games, conlanging, and law.