Arak, common in the Middle East. There is a similar flavour Greek drink, Ouzo. The Turkish Raki is similar. It's got Anise so tastes a bit like liquorish. The kinds I've seen are clear and go a little milky if diluted. And, um, tested/tasted! Some varieties are already milky, never tried those. Ouzo certainly tastes similar, but I don't remember if it goes milky. The Arabic or Hebrew could be transliterated as Ararat. Certainly there are a variety of Arabic pronunciations. Not all Arabs are Moslems and heated or boiled alcoholic drinks are permitted by some Moslem "denominations".
Lots of things have changed even since 1920s. Things that were common now sometimes unknown to most people. Some things are not even the same thing at all, like custard. Also Moonfleet was written about a 130 years after the time setting and the author had only books to research the period.
Edit:
Photo and description here:
https://en.wikipedia.org/wiki/Arak_(drink)#Consumption
In the interests of research I've compared Whisky, Whiskey and Bourbon. The Scotch can be slightly peaty and US burbon slightly soapy, maybe that's why Marlow drinks it with lemon (I think that's Whisky Sour). I've sampled commercial mead and brewed Metheglin (or at least one recipe) as well as stout, sloe wine (nothing like sloe gin), elderflower (white & sparkly) and elderberry (red dry "wine"). Research.