Namo
I suppose that most people reading this are familiar with the nembutsu, which in Japanese has been more or less standardized as namu Amida Butsu 南無阿弥陀仏. Learning Pali reminded me that the first word, namu in modern Japanese, goes back to namo on the subcontinent, and I was struck by the fact that the pronunciation namo appears in the Genji Monogatari's "Yūgao" chapter as well:
明がたもちかうなりにけり。鳥のこゑなどは聞えで、みたけさうじにやあらん、ただおきなびたるこゑにぬかづくぞ聞ゆる。 [...] なもたうらいたうしとぞをがむなる。
It would soon be dawn. No cocks were crowing. All they heard was an old an's voice as he prostrated himself full-length, no doubt for a pilgrimage to the Holy Mountain. ... "Hail to the Guide who is to come!" the old man chanted. (Tyler's translation)
So I got to wondering if maybe namo was the older form (borrowed directly from the source), and namu a later version (arising out of sound changes in Chinese and/or Japanese), but I wasn't able to find much information on the topic. According to the Nihon kokugo daijiten, the 1474 CE Bunmei setsuyō shū 文明本節用集 dictionary lists a whole bunch of alternate spellings: 南謨, 南芒, 南牟, 南膜, 南麼, 納無, 南莫, 南忙, 曩謨, 那蒙, many of which suggest a namo pronunciation, but there's no information as to which is older and, as the NKD says, exactly how sources written in kanji were pronounced is anybody's guess.
I suppose the key lies in finding out how 無 was pronounced in the relevant ancient Chinese period and region, but this lies beyond my abilities. Anyone got any ideas?
Carl:
According to my sources, the Middle Chinese for 無 was mju and the Old Chinese was *ma. Wiktionary lists the Wu reading as ム, the Han reading as ブ, and the Tang/Song reading as モ.
I don't think any of that really helps, but there you go.