That same student who revelled in the language of Juvenal (see earlier post http://teacherofclassics.com/?paged=2) came across this word in the Tenth Satire, line 138, where Juvenal lists some of the spoils and trophies of war, and says of them:

humanis maiora bonis creduntur. ad hoc se
Romanus Graiusque et barbarus induperator
erexit, causas discriminis atque laboris
inde habuit: tanto maior famae sitis est quam
 They are believed to be blessings greater than human. For this
the commander-in-chief, whether Roman, Greek or foreign,
exerts himself— from this he has his motive for peril and toil:
so much greater is the thirst for fame than for virtue.

The pupil had a question: why does Juvenal use the older form induperator, and in doing so break the convention that that the last word of a hexameter should be a trisyllable or less?  Is it done with a mocking tone?

The suggestion was interesting. Long words can be used mockingly in English— protoplasmic, sesquipedalian, plenipotentiary, etc.—in the Gilbert and Sullivan operas, for example.

But no: this does not seem to be a Latin trope, and the fact is that the word īmpèrātōr cannot be used in a hexameter, because of its lone short syllable. In a hexameter or pentameter, shorts come in pairs, except for the final syllable of line. And so a number of words cannot be used in any of their cases, including āppàrātùs, Vīmìnālìs, sērvìtūs, ēnìtēscō, and many others. And there are even more that cannot be used in some of their cases, like vīrgìnēs.

We find īndùgrèdī for īngrèdī in Lucretius IV 366-7, where he is trying, with moderate success, to explain the cause of shadows:

aëra si credis privatum lumine posse
indugredi, motus hominum gestumque sequentem…
 ..if you believe that air deprived of light is able
to travel along, following the moves and gesturing of people…

Lucretius was fond of using older formations: and indu is an older form of in. Furthermore, it can also be spelt endo—a reminder of the kinship with Greek. There are even examples of imperator being spelt endoparator.

Note: In the above examples, the macron is used for syllables that are long prosodically rather than vowels that are long by nature.