This is Bob Dylan's lyric:
"I got a letter on a lonesome day,
It was from her ship a-sailin',
Saying I don't know when I'll be comin' back again,
It depends on how I'm a-feelin'."
I wonder why he put an "a" before sailing and feeling?
This is an old form that is not used much, but survives in things like 'awake', 'asleep', etc. He's being poetical- you'll see these forms used in older writing.