It refers to the time of youth when one was naive and inexperienced, and therefore happy and optimistic - in other words, when one was "green," as in "unripe" or "not yet mature." It's a bit of a convoluted pun.
Like so many English idioms, the term was coined by Shakespeare in the 17th century (Antony and Cleopatra.)
One of the meanings of beg, according to the Oxford English Dictionary, is "to take for granted without warrant." The OED notes that this meaning is most common in the phrase to beg the question, and indeed all of the citations for this meaning are of similar, though not identical, phrases. Ignoring a few duplicates, here are those citations:
1581 W. Charke in A. Nowell et al. True Rep. Disput. E. Campion (1584) iv. sig. F f iij, I say this is still to begge the question.
1680 Bp. G. Burnet Some Passages Life Rochester (1692) 82 This was to assert or beg the thing in Question.
1687 E. Settle Refl. Dryden's Plays 13 Here hee's at his old way of Begging the meaning.
1852 H. Rogers Eclipse of Faith (ed. 2) 251 Many say it is begging the point in dispute.
The etymology of beg is no more helpful—in fact, it's hotly disputed. Some believe that it came from the Old English word bedecian, which in fact means "to beg," but that word has only been found once in all of the surviving Old English literature, and no clear links beyond its meaning and a very slight phonological similarity have been found between it and beg. Others say that beg came, via Old French, from the Latin begardus or beguin, a Christian lay mendicant order known in English as Beghards and Beguines. Either way, there's no clear connection to the meaning of beg used in beg the question.
What we're left with, then, is this: beg in this phrase means something like "to take for granted without warrant," but it only has this meaning in this and very similar phrases. It seems to have acquired this meaning sometime in the late sixteenth century, but how that happened is a mystery.
Best Answer
It should mean avoiding the question (or “to improperly take for granted”), when used in the original sense:
Begs the question:
(petitio principii, "assuming the initial point")
For a complete reference on the topic: fallacyfiles.
As Benjol mentions in the comments Eric Lipert has blogged about this:
Phil Koop commented on that blog post: