Jānussoṇi was a prominent brahmin who features several times in the suttas. The Dictionary of Pali Proper Names has a summary.
I’ve previously discussed this name, but my conclusion is I think incorrect there; I thought his name meant “lame knee”. But I can’t find any support for the reading Jānukṣīṇī.
According to the commentary Jānussoṇi is a title awarded the family priest (purohita) of Kosala, which would make him one of the most powerful brahmins alive.
Jāṇu normally means “knee”. Kaccāyana 671 derives this from √_jan_ (gamanaṁ janetītī jāṇu, “it causes movement, thus it is the knee”). It doesn’t strike me as hugely plausible, but what would I know? It is described as an unnadi derivation, in which the variant spellings n and ṇ are equally permissible.
Whether this is the same word as in the name Jānussoṇi is not stated by Kaccāyana.
The Chinese rendering 生聞 (T 125.2.665b18) translates as “Born Famous”, evidently taking jānu also from √jan and soṇi from the Vedic ṣvaṇi (“sound”, RV 1,058.04 etc.).
The latter derivation seems plausible to me, and, albeit indirectly, the Pali and Chinese sources agree on the root jan in the sense of “birth”.
Thus the Pali and Chinese scholars were on the right track, the root is indeed jan (= birth = those that are born = people), and the second element is “sound”. The sense is “famed among the people”.
It’s not easy to say if there is any connection between the Jānussoṇi of the Pali and the incidental references to Jānaśruti in the Sanskrit texts.
In the Satapatha Brahmana we hear of Aupāvi Jānaśruteya who offered sacrifice and went to heaven, only to return again, setting the model for all who must return again. He was, presumably, a famed brahmin and so might be the same person.
It is this same Aupāvi Jānaśruteya, evidently, whose opinions on the ritual were cited in the Aitareya Brahmana. (Translation at link).
tad u ha smāhopāvir Jānaśruteya, upasadāṃ kila vai tad brāhmaṇe: yasmād apy aślīlasya śrotriyasya mukhaṃ vy eva jñāyate tr̥ptam iva rebhatīvety. ājyahaviṣo hy upasado, grīvāsu mukham adhyāhitaṃ; tasmād dha sma tad āha
In the Chandogya Upanishad 4.1–3 we here of a generous king named Jānaśruti. Or at least the commentary calls him a king, although he is addressed as śudra in the text. Overhearing the gossip among swans (!), he seeks the teacher Raikva to learn more, in which he succeeds after offering his daughter. The commentary to the same book (CU 1.8) mentions him as a very learned person in recital.
It seems we can also establish the etymology clearly: jana (“people”) + soṇi (“sound”, “renown”, from Vedic ṣvaṇi, becoming śruti in Sanskrit).
The long initial a is due to it being a patronymic. Thus Kaccāyana’s unnadi derivation of “knee” does not apply here. Now, it is due to this unnadi derivation that the Pali word for “knee” has two spellings: jānu or jāṇu, reflected in the variants Jānussoṇi and Jāṇussoṇi. Most likely the original form was jāna, and jāṇu with its spelling variation was a contamination.
This suggests that we should prefer the form Jānussoṇi.
While we cannot say whether these legends, if they have any basis in fact at all, refer to the same person, we can at least establish that it is a known name; that the sense of the name is “famed among the people”; and that several accounts tell of a renowned, generous, and learned person, eager for knowledge, who shared the name of the Pali Jānussoṇi.
The Sanskrit texts do tend to argue against the Pali commentarial explanation that it was a title. Rather it was a patronymic used by the descendants of the original Janaśruti. It was probably used by several descendants over the years, making it impossible to determine the relationship between the various people of that name.