Except for the DN we find nijjhāna in a few contexts in the Suttas. In the AN stereotypically in this passage:
Mā anussavena, mā paramparāya, mā itikirāya, mā piṭakasampadānena, mā takkahetu, mā nayahetu, mā ākāraparivitakkena, mā diṭṭhinijjhānakkhantiyā, mā bhabbarūpatāya, mā samaṇo no garūti._
by oral tradition, by lineage of teaching, by hearsay, by a collection of scriptures, by logical reasoning, by inferential reasoning, by reasoned cogitation, by the acceptance of a view after pondering it, by the seeming competence, or because you think: ‘The ascetic is our guru. (trans. Bodhi, AN 3.65, AN 3.66, AN 4.193)
There is more diversity in the SN…
Aññatreva saddhāya aññatra ruciyā aññatra anussavā aññatra ākāraparivitakkā aññatra diṭṭhinijjhānakkhantiyā
apart from faith, apart from personal preference, apart from oral tradition, apart from reasoned reflection, apart from acceptance of a view after pondering it (SN 12.68, SN 35.153)
Yassa kho, bhikkhave, ime dhammā evaṃ paññāya mattaso nijjhānaṃ khamanti, ayaṃ vuccati –
‘dhammānusārī, okkanto sammattaniyāmaṃ, sappurisabhūmiṃ okkanto, vītivatto puthujjanabhūmiṃ
One for whom these teachings are accepted thus after being pondered to a sufficient degree with wisdom is called a Dhamma-follower, one who has entered the fixed course of rightness, entered the plane of superior persons, transcended the plane of the worldlings. (SN 25.1, SN 25.2, SN 25.10)
Tathāgatappaveditā cassa dhammā paññāya mattaso nijjhānaṃ khamanti. Ayampi kho, mahānāma, puggalo agantā nirayaṃ agantā tiracchānayoniṃ agantā pettivisayaṃ agantā apāyaṃ duggatiṃ vinipātaṃ.
And the teachings proclaimed by the Tathagata are accepted by him after being pondered to a sufficient degree with wisdom. (SN 55.24, SN 55.25)
And the MN provides some passages too
Tesaṃ te dhammā paññāya atthaṃ upaparikkhataṃ nijjhānaṃ khamanti.
Examining the meaning of those teachings with wisdom, they gain a reflective acceptance of them. (MN 22, obviously differently rendered than in SN 55.24)
atthaṃ upaparikkhato dhammā nijjhānaṃ khamanti, dhammanijjhānakkhantiyā sati chando jāyati
And how is final knowledge achieved by gradual training, gradual practice, gradual progress? Here one who has faith visits him; when he visits him, he pays respect to him; when he pays respect to him, he gives ear; one who gives ear hears the Dhamma; having heard the Dhamma, he memorises it; he examines the meaning of the teachings he has memorised; when he examines their meaning, he gains a reflective acceptance of those teachings; when he has gained a reflective acceptance of those teachings, zeal springs up in him; when zeal has sprung up, he applies his will; having applied his will, he scrutinises; having scrutinised, he strives; resolutely striving, he realises with the body the supreme truth and sees it by penetrating it with wisdom. (MN 70)
- MN 95,similar to SN 12.68 and MN 70
- MN 102, again similar to SN 12.68
I think for the moment we have to forget how Bh. Bodhi renders it and look purely at the context. From that I think we have two viable interpretations.
nijjhāna is indeed a kind of pondering, suggested by the rather depreciative context in the AN
we have here an alternative formulation of the jhana-process.
In the traditional formula we have sati --> vitakkavicara --> pitisukha.
Here the focus is not so much on samadhi, rather on dhamma. And taking the MN 70 passage we see
memorised (dhāreti = sati?) --> examine the meaning (upaparikkhati = vitakkavicara?) --> (ni)jjhāna --> zeal (chanda =piti?)
For now I prefer the second reading because in the SN and MN there is paññā involved, which is not trivial and usually reserved for higher functions. I think we might be on to something …