"absurd etymology," generally "anything illogical, outrageous hypothesis," 1711, from the Latin phrase, taken as the outstanding example of such an error. "A grove (lucus) [is so called] from not (a non) being light (lucendo, ablative of lucere "to shine;" see light (n.))." That is, it is called a grove because light doesn't get into it. This explanation is found in a commentary on Virgil (Aeneid 1.22) by Servius, a 4th century grammarian, among other places. Other ancient grammarians (notably Quintilian) found it paradoxical and absurd.
Modern scholarship, however, concludes that lucus and lucere probably do both come from the same PIE root (*leuk-) meaning "light, bright." De Vaan writes: "Lucus 'sacred grove, wood,' from PIE *louk-o- 'light place,' with cognates in Sanskrit loka- 'free space, world,' Lithuanian laukas 'field, land,' Latvian lauks 'field, clearing in the woods,' Old High German loh 'clearing' and English lea 'open field, meadow, piece of untilled grassy ground.' " Apparently the primeval notion in *louk-o- was a lighter place in a thick forest. Migration, change of climate, or felling of the woods might have shifted the meaning.