fulgur.

From Proto-Italic *folgos, from the same root as fulgeō (“flash, lighten”). The expected declension according to regular sound changes would be *fulgus /fulgeris. The nominative fulgus is in fact given by Festus as an alternative form. The oblique stem fulger- is attested in Lucretius ("quasi protelo stimulatur fulgere fulgur"), and also in inscriptions in the spelling of derived words such as fulgerātor. Some Romance descendants also derive from a stem fulger- (although they could alternatively descend from fulgere, the infinitive of the verb fulgō). The usual nominative/accusative/vocative singular form, fulgur, shows replacement of -s with -r, presumably by analogy to the oblique stem. This analogical change is not usual in neuter third-declension nouns, but rōbur n (“oak”), iubar n (“splendor; brightness”) and sulphur n (“brimstone; lightning”) are other possible examples. The usual oblique stem fulgur- shows replacement of expected -er- or -or- with -ur-. Kent 1932 and Parker 1986 suggest that -o- was raised to -u- by assimilation to the vowel in the first syllable in this word, and also in sulphur (though ulcus shows regular ulcer-). Compare also (non-neuter) augur.