New York Mike has given the answer:
5 : in the act of; : in the process of ;
*daddy's gone a-hunting*
A-hunting we will go, a-hunting we will go
Heigh ho, the dairy-o, a-hunting we will go
A-hunting we will go, a-hunting we will go
We'll catch a fox and put him in a box
And then we'll let him go.
The practice of putting a- before a gerund has long disappeared- or just about - and survives only in such songs like that above, and The Twelve Days of Christmas, and a few relics such as 'awaiting' (What's the T-shirt mesage?: "I am still awaiting public approbation for my arrival on earth."
However, what isn't commonly known is that ducks a-laying does not actually mean ducks - and that goes for turtle doves too. This is a Christmas song, and so has religious connotations. It is said that the song was written to help Catholics learn the rules about their faith when being a practicing Catholic in England was forbidden and could result in death.
1.True Love refers to God, Me refers to every baptized Catholic, and the partridge is Jesus.
2 Turtle Doves refers to the Old and New Testaments
3 French Hens refers to Faith, Hope and Charity, the Theological Virtues
4 Calling Birds refers to the Four Gospels and/or the Four Evangelists
5 Golden Rings refers to the first Five Books of the Old Testament, the "Pentateuch", which gives the history of man's fall from grace.
6 Geese A-laying refers to the six days of creation
7 Swans A-swimming refers to the seven gifts of the Holy Spirit, the seven sacraments
8 Maids A-milking refers to the eight beatitudes
9 Ladies Dancing refers to the nine Fruits of the Holy Spirit
10 Lords A-leaping refers to the ten commandments
11 Pipers Piping refers to the eleven faithful apostles
12 Drummers Drumming refers to the twelve points of doctrine in the Apostle's Creed
These days, we do rather take it in its highly-fanciful literal sense.