The Italians call it "Nocino", but it predates them by quite a bit.

Back in the days of the Celts and Druids, many seasonal rituals were observed, coinciding with various solstices and equinoxes.  One special day in their calendar was the Summer Solstice on or about the 21st of June, the longest day of the year.  As part of the festivities they would harvest unripe green walnuts, and steep them in spirits with select spices to transform over time.  Transform it did, oxidizing to a dark greenish-black color as the oils in the green walnuts interacted with the liquid for the next several months.  I don't know how a given enterprising Druid figured out that sticky and pungent green walnuts were something worth mucking about with, but muck they did, and the recipe carried forward to this day.

When I say "carried forward", I mean that those nosy Roman types developed a taste for Celtic Walnut Liquor, and took the finished product and recipe back to Rome.  The tradition in the Italian peninsula took root, and now the concoction is produced and sold there as "Nocino".  You can buy the stuff, but it ain't particularly cheap.  You can also make it yourself, which is what I attempted about 14 months ago.

A ckeaver is a good idea!

Assuming you have a source of unripe walnuts, you should be in good shape.  You'll also need a stout cleaver, cutting board, neutral spirits (Vodka, Moonshine, Everclear), a vanilla bean or two, cinnamon sticks, cloves, and the zest of one orange or lemon.  The quartering of the unripe walnuts is the biggest physical challenge - wear gloves, or be prepared to have walnut oil on your person as you cleave them forcefully.  You'll want to pack your quart or larger Mason/Bell/Kerr jar to about the shoulder with quartered unripe walnuts, at which point you'll shift gears.  (See the photo below of today's fresh batch in the Kerr jar)

With the small bit of space left in the jar, add a cinnamon stick, and one or two whole vanilla beans.  Toss in 10-12 whole dried cloves, and the cut-up rind of an orange or lemon. Then you get to add the solvent.  I used 190-proof Everclear, which guarantees that all the components will release their goodness to the mix in delightful fashion.  Top the jar right up to the brim, then secure it all with a new lid and ring.  Give it a good shake, then let it sit for a bit to allow bubbles work their way to the top.  If there's a lot of new air space under the lid, go ahead and top it off.  Otherwise, every day or so give the jar some sunlight, and if you can, open the lid to allow oxygen to interact with the mix for better color.

After just a few months, you'll have something that looks like SAE30 - it's that dark.  This is a desired transformation, as all the flavors do their thing in that alcohol.  You can sample, but remember that it's 190-proof high octane, so be careful.  When the day comes that you want to complete the process, strain the walnuts, cinnamon, vanilla, cloves, and orange peel pieces out of the mix, then run the liquid through a coffee filter or paper towel to get the remaining small bits out.  You'll have a greenish-black high octane liquid that will require cutting to a drinkable proof, and some sweetening with either honey or sugar to taste.  I cut mine to 80-proof, and use just enough sugar to leave a slightly bitter finish.

Congratulations - you've made Celtic Walnut Liquor, also known as Nocino!  While the origins don't look particularly appealing, you will be absolutely delighted with the results.  I liken it to a coffee liqueur, with a very prominent holiday nose.  I suggest serving your ancient concotion as an aperitif or digestif.  Enjoy!

Bottled, with a fresh batch in between awaiting transformation...