I think your second theory sounds more likely!

My take is that it's like afoot, abed, aplane, ahead, ahind, asunder, afoot, afire, acrawl, a-hunting: in, on, with, in the state of, in the condition of, in the process of

Vielleicht auch nur als Vokal für einen bessern Klang, einen besseren Rhythmus oder Ähnliches eingefügt.

Why not both?

I've also heard this song without the "a-" and it sounds just as good, mind you.

But anyway, "a-" does have that "present continuous" meaning. It's old English - perh. from Shakespeare's days, although I think it was used later in the USA. (see http://www.mwg.org/production/websites/jackta... for example)

You can only use it with nouns starting with consonants - you can't say a-eating or an-eating.

I wonder if it is similar to the German "ge-" in "Er kam geritten".
In Bavaria you say "o-" like in "Obatztes", perhaps it's from there?

@Archfarchnad: can't quite agree with your etymological observations: "geritten" is surely just the past participle: I see no similarities with "a-riding". Bayr. "Obatztes" -- the "o" is AFAIK the prefix "an-" -- "angebatztes".

Using a-riding and a-giving means that each line in the poem has seven syllables which helps the rythm. When sung the tune I presume takes care of the problem so that the 'a-' can be dropped.
Just a thought, so don't go a-stompin' up and down if you don't agree.

Robert Herrick
To the Virgins, Make Much of Time

Gather ye rosebuds while ye may,
Old time is still a-flying,
And this same flower that smiles today,
To-morrow will be dying.

The glorious lamp of heaven, the sun,
The higher he's a-getting,
The sooner will his race be run,
And nearer he's to setting.

That age is best which is the first,
When youth and blood are warmer,
But being spent, the worse and worst
Times still succeed the former.

Then be not coy, but use your time,
and while ye may, go marry,
For having lost just once your prime,
You may for ever tarry.

It definitely has to do with metre. Seems to be a rather common method.

I still ask, why not both??

Apparently, a- as a prefix comes from "at".

Webster's unabridged also tells us... (at http://dict.die.net/a/)

A In process of; in the act of; into; to; -- used with
verbal substantives in -ing which begin with a consonant.
This is a shortened form of the preposition an (which was
used before the vowel sound); as in a hunting, a building,
a begging. "Jacob, when he was a dying" --Heb. xi. 21.
"We'll a birding together." " It was a doing." --Shak.
"He burst out a laughing." --Macaulay.

Note: The hyphen may be used to connect a with the verbal
substantive (as, a-hunting, a-building) or the words
may be written separately. This form of expression is
now for the most part obsolete, the a being omitted and
the verbal substantive treated as a participle.

My etymological wonderings were only wonderings, and probably nonsense, but why is "geritten" only a past participle if you can say "er kommt geritten"?

What do you mean? The "geritten" in "Er kommt geritten" is past participle.

Perhaps I just don't understand the German - I thought it meant "he comes along riding a horse".

@Archfarchnad: Well, it may seem "ungrammatical", but it is definitely common usage in German - but only with such verbs of motion as "laufen", "rennen", "reiten" etc. in combination with "kommen": "er kommt gelaufen / geritten" etc., basically indicating a movement toward the speaker.

What's interesting, though, is the fact that in these instances, the past participle form often takes the ("prepositional") prefix "an-":
"er kommt angeritten" etc.
This would make the interpretation of the prefix "a-" as a derivative of the preposition "at" seem plausible ...

Wow, German grammar which seems illogical, what a novelty! I thought that was English's job...

The "at" thing came from sentences like:

He's at hunting (this form is no longer used)

which I suppose meant "Er ist beim Jagen".

"Er kommt geritten" seems more like present progressive to me.

But from my experience the a- is just old-fashioned and was there to emphasise the progressive tense or maybe just to make it sound a little nicer. At least that's how I use it.