We can finally end this discussion now that I, the Order's resident Latin scholar, is here to bestow upon my humble and expansive knowledge of the universe.
William Marshal is the Anglicized version of the Old French name (and title), Willame (or Guilliame) le Mareschal. Mareschal (a masculine French noun) stems from the Old Frankish "marhskalk" which means "Stable keeper". Bear in mind, this is a title, not a name. The late Romans had a similar title, "Comes Stabuli" or "Count of the Stable" from which we get the English derivative "Constable". In translating this title, we are presented with two options: use the literal Roman title, or latinize the French title.
Because William was not Roman, giving him the Roman title would be inappropriate. There is no precedent that I have seen in medieval scholarship to correlate titles. Instead, his title would have been Latinized to the Medieval Church Latin, rather than to the Roman Classical or Vulgar Latins.
In this situation, using the third declension would be most appropriate. The third declension is the Latin "Island of misfit toys". Irregular nouns and adjectives all get dumped in the third declension. (Irregular verbs get dumped into the third conjugation). It's the most flexible in terms of its construction.
In Latin, we cannot use direct translation of the English words, as there are no words for "of" or "the". And instead of using word order to determine a word's role in a sentence, Latin uses endings.
In the (regular, masculine/feminine) third declension we have the following endings
Singular Plural
Nominative - -es
Genitive -is -um
Dative -i -ibus
Accusative -em -es
Ablative -e -ibus
Vocative - -es
Since "Order" is the subject, we'll give it Nominative ending (which is none at all). We end up with Ordo
The next part is "Marshal", the tricky word. It is masculine in Old French, so we're safe giving it a masculine gender in Latin. Therefore, we can use the regular third declension endings instead of the neuter or irregular i-stem endings. That gives us an ending of "-is" - so we get Marshlis
Voila!! Order of the Marshal = Ordo Marshalis
Because I FUCKING LOVE LATIN, we're going to translate the motto, too! Besides, every cool country and organization has a Latin motto, why not us?
"Chivalry our Strength, Brotherhood our Sword"
This isn't a full sentence, but a fragment. However, a verb of being ("is") is implied: Chivalry (is) our Strength, Brotherhood (is) our Sword. In our translation, we're free to include or exclude this implied verb. I'll exclude it. We'll be using the first and second declensions for this one, so those are given below:
First declension
Singular Plural
Nominative -a -ae
Genitive -ae -arum
Dative -ae -is
Accusative -am -as
Ablative -a -is
Vocative -a -ae
Second declension
Singular Plural
Nominative -us -i
Genitive -i -orum
Dative -o -is
Accusative -um -os
Ablative -o -is
Vocative -e -i
"Chivalry" presents us with our first problem. There's no Latin word for it, because the Romans didn't have that concept. The most relatable concept is "virtue" or virtus. This comes from the word vir, viri m. meaning "man". Virtue literally means manliness according to the Romans. Other words could be nobilitas or magnanimitas, however neither of those quite conveys the proper sentiment. So virtus, virti m. it is. This is a regular, masculine second declension noun.
"Our" is the possessive form of the word "we". In Latin, this translates to the adjective "nostrus, -a, -um". It is either first declension feminine, second declension masculine, or second declension neuter based on the noun it will be modifying. Adjectives and nouns must always agree in gender, number, and case. Before we can translate "our", we need more info from the next word - "strength".
"Strength" is another tricky word. Latin has several words that could mean "strength". There's vis which means strength (physical), power, violence, virility. There's potestas which means strength, ability; rule, force. There's robur potency, force, rigor, robustness, vitality. There's nervus which means sexual prowess, masculine virility, power, and force (and is Latin slang for penis) - we'll skip this one. And there's fortitudo which means courage, fortitude, stability, strength, standfastness. That last word conveys the best sense of what this Order is all about, while the others have more physical and violent connotations. Fortitudo, fortitudonis f. is a feminine third declension noun. This means that "our" will also have to be feminine (even though most of the order are male). the feminine form for "our" - nostra - falls into the first declension.
Chivalry will be masculine singular nominative(subject), while "our" and "strength" will be feminine singular accusative (direct object). The translation of the first half will be:
Virtus nostram fortitudonem
Using the same process we can get the second half:
Brotherhood our sword
"Brotherhood" also has several meanings in Latin. There's collegium but that has a societal connotation. There's contubernium but that literally means "tent mates" (we're not on campaign together, so we'll skip this one too). Congregatio and hetaeria both have more tangible connations, whereas our motto implies an ideal or a virtue of brotherhood. So we'll go with fraternitas, which like fortitudo is a feminine third declension noun. (hint: third declension feminine noun stems end in -s, -o, or -x)
We already know how to translate "our" so on to the next word - "sword". This one's easy - gladius! It's masculine second declension.
So we'll use the same cases as before to translate the second half.
The motto in Latin reads:
Virtus nostra Fortitudonem, Fraternitas nostrum Gladium
There you have it!