I had a "Big Brother" (United Way) when I was around 10y/o. He was from England and a truck driver here in the US so his handle was Rainman.
He was teaching me C.B. etiquette and gave me the handle Rainman Jr.
Once the movie Rainman caught on, I got a bit older, and I got online (1200 baud bbs around 12-13 y/o), people were associating my nick with the character in the movie and it began to annoy me.
Around 14 y/o I found out that Abba was Hebrew for father.
One day it just sorta came to me like this: Abba = father, father of the rain, rainabba and BAM, there it was.
Since then I've used it as my well-known handle on the net.