ziesecharles7040-san, エリックさん、マイケルさん、
true; じゅーぶん is NOT the common way to write じゅうぶん or 十分
We often use this "bar" (ー) with hiragana in commical way. From the sentence, I sensed the author
used this as a cynical meaning, or "accusation".
すでにじゅーぶん喧嘩腰である。 sounds cynical and has a hidden connotation of "what are you talking about? You already ARE hostile..."
I don't know if this sentence in English can be a good translation becasue I don't know the context,
but if the character pretends or insists s/he is not provoking argument or fight and someone says すでに・・・ sentence,
it'd be pretty much correct way to understand. If there's no such context behind, it could be like "uh-oh; here we go again... s/he is already enough hostile..." or something like this
The "commical" that I mentioned is because we don't use this ー as a part of hiragana word (it's used only with katakana
originally). So, intentional misusage for us has "funny", "commical" and/or "casual" connotaion. Similar thing can be
observed in hiragana words which are supposed to be written in katakana.
Natsuko(奈津子),
Team JapanesePod101.com