A friend recently asked me: 'What do you call an arbitrary word / sound that replaces another word / concept (thingamajig, watchamacallit and gizmo, or words found often in pop songs, 'my ding-a-ling, my ding-a-ling, I want you to play with my ding-a-ling', or words like bling)'

I am unable to answer him - any help? I am not certain that the examples he gives all fall within the same category, but a lead on this would be gratefully received.

http://thesaurus.reference.com/browse/slang

It's tedious and not always helpful, but you might try OneLook Reverse Dictionary, eg:

http://onelook.com/?w=*%3Ajargon+slang&ls=a

http://onelook.com/?w=*&loc=revfp2&clue=word+sounds

These may be too specific, but all I can come with are patter and doo-wop.

Thanks - what he is looking for is the definition of such words - what does one call this category of words, technically? They are not Onomatopoeic - what are they?

In the song, the ding-a-ling is (supposedly) a bell, so that's an onomatopoeia since bells make that sound (the euphemistic meaning in the song is something that dangles like a bell). Bling (shortened from bling-bling) is also more or less an onomatopoeia - the cartoon sound of light glistening off gold/gems or the sound of lots of jewelery clinking together.

thingamajig, watchamacallit, gizmo, etc. are often called placeholder names. A rather extensive list in multiple languages can be found here : http://en.wikipedia.org/wiki/Placeholder_name

Some doo-wop lyrics can be placeholders (Who put the bomp in the bomp-shu-bomp-shu-bomp?) because they are taking the place of other words, but some are merely exclamations or a possibly form of scatting (Be-bomp-a-loo-bop, a-whamp-bam-boom) or maybe early beatboxing.

and jazz vocalists "scat".

perhaps vocalese?

Rut-roh, Myridon, I think you just got mantled.

Vocalese is somewhat the opposite - putting lyrics to an instrumental piece.
---
Why did Ludwig write this silly tune?
It wasn't free.
That's for sure.
The money, all in cash,
Came from Elise.
Thus it was she
He wrote it Für.

good point, Myr.

A bell? I don't think so.

placeholder names

I would've called them metasyntactic variables (e.g., foo, bar, baz, and quux) from the term in computer science, but placeholder name works fine for me.

honi soit qui mal y pense
o (halo)

"(supposedly)". I also know a Friendly Little Cat. Or perhaps you'd like a second-hand chair...

If I can't sell it, I'm gonna sit down on it.
I ain't gonna give it away.
Now darling if you want it, you're gonna have to buy it.
And I mean just what I say.