轰 can be found in some dialects, often with a 个 found on the end, to mean what.
This is most definitely a contraction. (Similar dialects also feature a contraction of 哪 and 样 as 孃 niang, also meaning what.)
What could 轰 be a contraction of?
My best guess at the moment is that it is a contraction of something like 何 + 用, but that's a stab in the dark at best.
To answer @dan's questions in the comments:
What dialects use it?
I believe it is, perhaps too specifically, part of 仁富小片.
Do you have a specific example?