old time is a-flying

Old time is a-flying.

What is "a-" for in this word?

Thanks

Jason

.
It means 'in the process of'.

Quote:

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a-

2. (literary (or archaic) used before a present participle - in the act or process of ⇒ come a-running, go a-hunting

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(Collins)

Rover

Note the words "literary" and "archaic" in that definition. Don't start trying to use this construction in everyday speech or writing.