By Jove!

[iwbus]

So who or what is Jove?

[jenstar]

The Roman god, Jupiter - the bringer of joy, hence the word "jovial".

[Quizmonster]

Just to add a little to Jenstar's excellent answer...the god of the ancient Romans was originally named 'Jovis'. This was later compounded as 'Jovis-Pater' (Father of Jovis) and then - by telescoping these words together - as 'Juppiter/Jupiter'. Apart from being the joy-bringer, he was their supreme deity - the god of gods. To swear 'by' Jove, therefore, was to offer the mightiest of oaths and first appeared in English in the 16th century. There was also believed in those days to be a connection, because of the sound, with the name 'Jehovah'... God as presented in the Bible.

[Quizmonster]

I should, of course, have written 'Jovis the Father' rather than 'Father of Jovis' in the brackets above.

[FredPuli]

Easy mistake to make QM. The genitive singular if Jovis meaning 'of/ belonging to Jovis' is Jovis too ! Don't recommend it, for the shade of meaning, as great Latin but technically correct.