It's not often you see verjuice in the popular press:
The best substitute is probably the sour grape juice sold in Persian
markets, which I suspect is basically the same thing. Otherwise, modern cooks
typically use lemon juice or vinegar in its place.
Le Grand claims that it was originally made from the juice of sorrel,
http://books.google.com/books?id=xY1AAAAAcAAJ&pg=PA146&dq=inauthor:Aussy+verjus&hl=en&ei=Y4SMTrL6JY73sQKLrbS2BA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDUQ6AEwAA#v=onepage&q&f=false
but
in fact there are earlier references to a specific grape called "verjus" and to
rents paid in it. The Romans had a similar product called "omphacium" (though
that could also refer to a kind of oil made from underripe olives). But this did
not seem to be the ancestor of verjuice - Charlemagne, who had his stewards keep
garum on hand, says nothing about either omphacium or verjuice. Verjuice does
not seem to appear (like trenchers) until about the 12th century.
Though when I first translated my own version of the Viandier I thought of
it as an archaic ingredient, in fact it has never fully disappeared. It's
mentioned fairly frequently in 18th century cookbooks and still, if more
sparingly, in the nineteenth century.
I'm probably not the only one (with Le Grand) to be frustrated by trying to
puzzle out what "verjus de grain" referred to. In fact, it appears to be an
equivalent to the grape version, but made with beer, as "verjus de pomme" was
made with cider. [UPDATE 8/23/13 : Terence Scully says this was probably a kind of jam made with preserves of the same kind of tart young grape used to make the juice; that's a creditable explanation, especially since the one early source for "verjus de grain" itself seems to be period guesswork.]
The word "agresta" is more or less contemporary with verjus and
was the southern French or Italian (as well as Latin) word for it;
omphacium is the Roman word for something similar, but from an earlier
period.
For those who care to puzzle through the Latin, Du Cange cites an author who specifically distinguishes between agresta and omphacium:
http://books.google.com/books?pg=PA150&dq=agresta+omphacium&ei=ioGMTsK6NeilsALCpLCpBA&ct=result&id=eeI-F_-b7xIC#v=onepage&q=agresta%20omphacium&f=false
As I understand, this comes down to using verjuice as opposed to omphacium, depending on the species being cooked.
For those who care to puzzle through the Latin, Du Cange cites an author who specifically distinguishes between agresta and omphacium:
http://books.google.com/books?pg=PA150&dq=agresta+omphacium&ei=ioGMTsK6NeilsALCpLCpBA&ct=result&id=eeI-F_-b7xIC#v=onepage&q=agresta%20omphacium&f=false
As I understand, this comes down to using verjuice as opposed to omphacium, depending on the species being cooked.
The Romans also had a kind of olive oil equivalent - oleo gremiali - made from very young grapes. Further research may yet uncover other liquids made from young, tart fruits (yes, the olive is a fruit).