I've got this entry from 《广安方言与民俗词典》which goes as follows:
□ is as Wiktionary states:
Used as a placeholder for an unknown hanzi.
There's a good chance though that this character word does have an actual representational character.
I've not been able to find any MSM fei readings that mean to throw up/to barf, etc.
Notes: fei3 is equivalent to [fei⁵³] - so almost the same as fei4 in MSM.
Any ideas on what it can be?
My leaning at the moment is that fei3 is actually a contraction (合音) of something like 发呕 or 发哕 - not that I have any proof for it but it seems to be the most likely at the moment.