Laudator Temporis Acti

Dave Lull brought to my attention a new translation of Isidore's Etymologiae, The Etymologies of Isidore of Seville edited by Stephen A. Barney, W. J. Lewis, J. A. Beach and Oliver Berghof (Cambridge, 488pp, £85, ISBN 0521837499), and a review of the translation by Dot Wordsworth in The Spectator (5 August 2006) under the title Shedding light in dark places. The review ends with these interesting paragraphs about the etymology lucus a non lucendo (the word grove, "lucus," comes from not being light, "a non lucendo"):

As for lucus a non lucendo I have heard it attributed to Varro (116-27 BC), though I cannot find it in his surviving work. (It is certainly to be found in the fourth-century commentary on the Aeneid by Servius, one of Isidore’s authorities.) Doubts about its truth appeared early. ‘Are we to admit the derivation of certain words from their opposites, and accept lucus a non lucendo, since a grove is dark with shade?’ asked Quintilian (AD 35-100).

The last laugh is that a present-day scientific etymological dictionary of Latin like Ernout and Meillet does find a connection between lucus, no matter how dark it is, and lucendo. Both come from the Indo-European root leuk- that gives us Old English leah, ‘wood’ or ‘clearing’, as well as Latin lux, lumen and luna. Isidore was brighter than his shady detractors.

I don't have access to Ernout and Meillet, but Julius Pokorny, Indogermanisches etymologisches Wörterbuch (Bern: Francke, 1959), s.v. leuk-, supports the connection between lucus and lucendo:

lūcus, alat. Akk. loucom 'Hain', eigentlich '(Wald-)Lichtung' (vgl. collūcāre 'in einem Wald eine Lichtung vornehmen', interlūcāre 'Bäume auslichten').

I too was unable to find lucus a non lucendo in Varro's De Lingua Latina. Don't be misled by H.E. Butler's translation of Quintilian (1.6.34) into thinking that the phrase occurs there:

But are we also to admit the derivation of certain words from their opposites, and accept lucus a non lucendo, since a grove is dark with shade, ludus in the sense of school as being so called because it is quite the reverse of "play" and Dis, Ditis from diues, because Pluto is far from being rich?

Quintilian's Latin reads:

etiamne a contrariis aliqua sinemus trahi, ut "lucus" quia umbra opacus parum luceat, et "ludus" quia sit longissime a lusu, et "Ditis" quia minime dives?

As Wordsworth points out, the phrase seems to originate with Servius, in his commentary on Vergil's Aeneid (1.22):

et dictae sunt parcae kata antiphrasin, quod nulli parcant, sicut lucus a non lucendo, bellum a nulla re bella.

Here are passages from Isidore's Etymologiae where he refers to the derivation.

1.29.3:

Sunt autem etymologiae nominum aut ex causa datae, ut "reges" a [regendo et] recte agendo, aut ex origine, ut "homo" quia sit ex humo, aut ex contrariis ut a lavando "lutum" dum lutum non sit mundum, et "lucus" quia umbra opacus parum luceat.

1.37.24:

Antiphrasis est sermo e contrario intellegendus, ut "lucus" quia caret lucem per nimiam nemorum umbram; et "manes" id est mites (quum sint inmites) et modesti, cum sint terribiles et inmanes; et "Parcas’ et "Eumenides" Furiae quod nulli parcant vel benefaciant.

14.8.30:

Lucus est locus densis arboribus septus, solo lucem detrahens. Potest et a conlucendo crebris luminibus dici, quae ibi propter religionem gentilium cultumque fiebant.

17.6.7:

Lucus est densitas arborum solo lucem detrahens, tropo antiphrasi, eo quod non luceat; sive a luce, quod in eo lucebant funalia vel cerei propter nemorum tenebras.