Gules means red in heraldry. It has meant red since 1165 at least, in French (where the word is gueules). In early times, other words were also used to mean red, such as "rouge" (red), "vermeil" (vermilion) and "sinople". This last word comes from the city of Sinope in Asia Minor, where the local clay had a red-ochre color (this is similar to the modern use of the word sienna). Until the mid-14th c., French blazon called green as Anglo-Norman blazon does, i.e., vert. Then, the word sinople switched from meaning red to meaning green, for obscure reasons (perhaps to avoid confusion between vert and vair, which sound the same in French).
The problem with gules or gueules is the etymology. The French word looks like a plural of the word gueule, from medieval Latin gula, meaning a carnivore animal's mouth. But what could the link be? 19th and 20th century dictionaries and reference works see no link, and have tended to adopt a derivation from the Persian word gul, which means rose (the flower). This hypothesis has been proposed since the 17th century at least, but there seems to be no firm evidence in its favor.
Another derivation of gueules, the color from gueule, the mouth, has also been proposed. It is argued that the word gueules originally (as of the mid-12th c. at least) designated small strips of fur used as trim on fur coats, around the neck and wrists. It appears that in some cases the "goules" are made of marten fur (one text says "goules de martre"); and that, even in the early 20th c., it was customary to use the inside of the throat of the animal. At that time, the word gueule already meant mouth.
The one problem is that this fur is a yellowish-brown color. In one text, an ermine skirt is described as "circa collem et circa manus rubris gulis praeparatam." It is possible that these fur trims were commonly tinted red, although there is something a bit circular in this hypothesis.
In any event, a word derived from furs would not be surprising in heraldry, since vair, ermine and sable are also names of furs. But then again, an oriental origin, although there is no proof in this case, would also be in line with the etymology for azure and sinople (vert in French).
Here is the Oxford English Dictionary's note on the etymology of gules:
Old French goules, gueules (Fr. gueules) = med. Latin gulae (plural), ermine dyed red. The ulterior etymology is disputed: the word coincides in form with the plural of the French and medieval Latin word for `throat'. If the heraldic sense be the original, the allusion may be to the colour of the open mouth of a heraldic beast. It seems more likely, however, that the heraldic use is transferred from the sense `red ermine', in which case the word may represent some oriental name; but the suggestion of derivation from Persian gul, rose (Hatzfeld-Darmesteter), is very improbable.
Notice that the OED finds it "more likely" that the heraldic use came from the furs rather than the reverse; but they do not seem to be aware of, or interested in, the possible connection through furs from throats of the marten, since they think that gules is ermine died red: in which case the origin remains a complete mystery (what would the connection be with throats of animals?) and they see an oriental origin as possible.
Anybody know a furrier?...
K. Nyrop, _Gueules, histoire d'un mot_, Romania (1922) 68:559-70
I was recently reading an article On the Etymology of Gules in the proceedings of an SCA heraldic symposium held in 1981 in Denver, Colorado. (I'm afraid it's probably out of print.)
The author made a convincing argument (to my amateur eyes) using phonetics, that gules descended from the Latin gula (accusative plural, gulas), through Anglo-Norman, to French and modern English. Her contention was that, if it descended from a Persian or Arabic word, its sound would have undergone pronunciation changes (the g would have been given a j sound, the vowel quality was altered). These changes in Old French occurred earlier than the earliest recorded borrowed Arabic words (late 11th Century).
The author was identified only by her name in the SCA: Marina of Eastcliffe, living (at that time) in the American Mid-West. Some of the sources referenced were: