"Is with me coffee"
"gen i" is from "gan" (with) + "i" -- the original form was "gennyf" where the -a- turned into -e- under the influence of the -y- in the next syllable; when the final -f dropped as it does so often, "genny" sounded pretty much like "gen i" which I suppose is why it is spelled like that now; people reanalysing it as "gen" + the word "i" which means "I".
Diolch! That is what I was thinking but I wasn't sure. I think my Irish studying has helped me with some of the structures in Welsh. I love how prepositions are used in both!
Why can't this be "Mae coffi gyda fi", as in the same structure when saying "I have a cat" (Mae cath gyda fi)?
In colloquial Welsh, the two different ways depend on where you come from. In general:
North Wales: Mae gen i ...
South Wales: Mae ... gyda fi
They mean the same thing: "I have ...".