It suggest to me the sort of toddler mispronounciation of "Grandpa" which is then taken up by the family as an affectionate term. Toddler misrpronounciation as "Grumpy",however, would probably not be adopted.

The only possibility that suggests itself to me is some connection to The Deerslayer or Last of the Mohicans.

Well, Sid Schwab managed to turn Mildred into Moomump for whatever that's worth.

I've never used anything but 'farmor/-far' and 'mormor/-far' - never 'bedstefar/-mor'. Oddly enough I've almost always been on a firstname basis with my parents which I've since come to realise is rather unusual.

I'm from Texas and I've never heard Bumpy before.

Well, my youngest niece calls my dad "Pocky," which sounds like a related concept. (He wanted to be called Grandpa, but alas.)

I'll let you all know in a year or two.

The only "Bumpy" I've ever heard of is Bumpy Kanahele, a Hawaiian nationalist who opposes the Akaka bill.

My father is now known as Pumper because this was how his first grandchild rendered the word "Grandpa".

The grandfather of his wife (my step-mother) was known as Bumper for exactly the same reason.

We are English, my step-mother is Scottish.

I presume in terms of developing speech production in babies "b" and "p" sounds come before "g" sounds.

But what about grannies? Are there any Bannies? Pannies? I've never heard of any. Strange.

A schoolfriend of mine (south UK) called his grandfather 'Bomper'... for similar reasons as above.

But what about grannies? Are there any Bannies? Pannies? I've never heard of any. Strange.

Yes my great grandmother was always known as Banny. She would've been 3rd generation Australian of Scottish extraction.

I've always considered it to be derived from an ancient toddler mispronunciation of "Granny".

The Houston Oilers' coach in the 1970's was "Bum" Phillips; the name was glossed as a clip of "bummer," his baby sister's version of "brother."

I remember reading a bit a while back on the typical development of early-childhood phonetic discrimination/production -- specifically the notion that a very rudimentary initial set of sounds (one plosive for all of "p" and "b" and "g" and "t") trees out over a period of months/years into increasingly specific individual sounds -- and that got me thinking about the variants people have reported over on askme.

Since "that's how the kid mispronounced 'grampa'" seems like such a common, plausible explanation, it'd be interesting to see someone put together a model of e.g. Likely Nicknames For Grandpa based on what's known about phonetic development. (As well, perhaps, from what's known of adult biases in lay interpretations of Cute Kid Speech.)

The variation "Bompa" is also to be found at
http://www.fact-archive.com/dictionary/Grandfather

My brother and I called our grandmother BamBam becaue I couldn't say Grandma. Grandpa was just Grandpa, though, and I have no idea why. I doubt I could say it right, but no nickname stuck with him. I and my parents are from California, but BamBam was from Arkansas.

My daughter calls her (very good natured) grandfather "Grumpy" which he seems to like. It stems from very early mispronunciation.... I've never heard "Bumpy" before.

Josh Millard: There's also another issue tangled up with the developmental issue; Given an intermediate phoneme produced by a child, how are the adults likely to interpret it?

My greatgrandfather was known as Bumpa; he was a Danish immigrant. He and his grandchildren lived in the Boston area.

A whole compendium of grandpa and grandma terms here:
http://www.banananana.com/ReadNames.asp

(And, I'm sure you've heard the line that goes, "If I had known how much fun grandchildren are, I would have had them first.")

I'm from Texas and my niece and nephew call my dad "Bumpy." It came from my niece not being able to say "Grampa." For a while, he was called "Grumpy," which we all thought was hilarious. Though I'm from Texas, this is the first time I've ever heard of someone else calling their grandfather "Bumpy." Who knew?!

I've never heard of "bumpy"--I called my grandfather Opa (they came from Germany).

David: exactly, yeah. That's part of what I was (poorly, vaguely) handwaving toward with "lay interpretations of Cute Kid Speech".

(Jumping tangent from naive to willful mis-understanding of phonetic output, I wonder if lhat has covered the Hatten är din thing?)

My childhood friend has a young daughter (about 7 now) who refers to her grandfather as "Buppy"--like "puppy", only with a B. Not quite "Bumpy", but pretty close. It was explicitly described to me as "how she pronounced it when she was little". This is in western Pennsylvania.

The only "Bumpy" I've ever heard of is Bumpy Kanahele, a Hawaiian nationalist who opposes the Akaka bill.

There's also Bumpy Johnson, Harlem mobster and cameo character in "American Gangster" - so called, though, because of a bump on the back of his head.

My little brother as a child called our grandfather "bee-pah". That was an invention of his, as far as we're aware. It persists in the family as an affectionate term here and there, but he mostly abandoned it as he got older.
He also rendered "Luke" as "Doo". I guess it was the best he could do at such a young age :)