lucus a non lucendo
[it is] a grove by not being light
Notes
From late 4th-century grammarian Honoratus Maurus, who sought to mock implausible word origins such as those proposed by Priscian. A pun based on the word lucus (dark grove) having a similar appearance to the verb lucere (to shine), arguing that the former word is derived from the latter word because of a lack of light in wooded groves. Often used as an example of absurd etymology.