Monks were heavily involved in distilling liquor during the middle ages in Europe. This was because there the monasteries were perhaps the biggest centers of learning at the time, alchemy was a major branch of contemporary knowledge, and distilling was an important technique in alchemy.

Thus monks were among the only people in medieval Europe who knew how to distill. Furthermore, the monasteries were big producers of wine and beer. It was a natural progression for the monasteries to start running wine or beer through their stills to produce distilled liquors. Initially they had probably experimented with distilling in an effort to solve alchemic puzzles such as turning lead into gold. They never did solve that one, but transforming ale and wine into whiskey and brandy was a pretty good consolation prize.

The South of France, Switzerland and Ireland were all major centers of monasticism and distilling. The association between the Swiss city of Geneva and distilling would be where the term ‘geneva liquor’ comes from.

I do not know what the original ‘geneva liquor’ would have been, though a clear, unaged, wine based brandy seems a likely guess. It might have been sweetened and flavored with herbs in the manner of monastic liqueurs like Benedictine and Chartreuse.

Just for the sake of interest, the oldest monastic liqueur currently in production is Benedictine, the formula for which supposedly dates back to 1510 - though realistically it has probably changed over the years. The 12th Century is quite a bit earlier than that, so it is hard to know exactly what type of liquor monks were distilling then.