As far as I know, adjectives modify nouns. So we can say "American gum", "Australian gum", etc. Why then do we say "gum Arabic" instead of "Arabic gum" ? Thanks in advance.
Gum Arabic (also gum arabic) is the gum or sap of some kinds of acacia trees.
The Oxford English Dictionaty defines it as
A water-soluble gum exuded by certain acacias (esp. Acacia senegal), used in the food and cosmetics industries and in glue and incense.
The reason it is gum arabic and not arabic gum is presumably due to at least two things:
1 It is referred to in other languages as gumme arabic (French), gomma arabica (Italian), gommi Arabicum (Latin). So it seems English writers chose to use the word with the noun gum first followed by the adjective.
The kind of gum that you chew is short for chewing gum. So you could refer to some as 'American (chewing) gum', 'Australian (chewing) gum' and even 'Arabian (chewing) gum'.
Note: Arabian is the adjective that has much broader usage, as in Arabian knights, Arabian horses, Arabian coffee, Arabian mustard, etc., whereas 'Arabic' usually refers to the language, writing or numbers (ie 'arabic numerals').
Thanks to the OED online for much of this information.