There’s a common Pali word acchariya, which means “amazing, wonderful.” We find it sometimes in a negative form an-acchariya, where it takes the sense “unsurprising, not unusual”.
But it also occurs in a stock phrase, usually where a verse or simile occurs to the Buddha:
Apissu bhagavantaṃ imā anacchariyā gāthāyo paṭibhaṃsu pubbe assutapubbā
And then these anacchariyā verses, not learned before in the past, occurred to the Buddha.
Now, given that this usage is highly specific to this context, surely there must be some meaning in the usage here. In his more recent translation at SN 6.1, Ven Bodhi follows the commentary in taking this as a mere intensive of acchariyā and rendering “astounding”. In the Majjhima translation, he had used “spontaneously”. There’s a substantial discussion of this point in note 365 of the Connected Discourses, which I won’t repeat here. Suffice to say that Ven Bodhi chooses astounding only “for lack of a better alternative”.
Given, however, that the sense “unsurprising, not unusual” is well attested and unproblematic, it would avoid semantic proliferation if this applied here, too.
Now, while we usually take acchariya in the sense of “amazing”, it blurs into what we might call “supernatural, magical”. In this stock phrase, this sense works well. Remember, the Vedas were traditionally held to have been transmitted by divine inspiration, and it is essential to the Buddha’s approach that he establish his own authority greater than that of the gods. Thus we could paraphrase:
I didn’t get this from any supernatural means, from any divine inspiration (unlike the Vedas), nor did I learn it from anyone else.
It’s not so easy to say how to translate. Since the Vedas were held to be channeled from the gods, we could render it “unchanneled”. We could also use “uninspired”, although obviously not in the everyday sense of the word. Perhaps its best to make the sense more explicit:
And then these verses, which were neither supernaturally inspired, nor learned before in the past, occurred to the Buddha.
There’s an interesting case in MN 125, where both the sense of anacchariya occur in their normal idiom, but next to one another. With Ven Bodhi’s translation:
Sace kho taṃ, aggivessana, jayasenassa rājakumārassa imā dve upamā paṭibhāyeyyuṃ, anacchariyaṃ te jayaseno rājakumāro pasīdeyya, pasanno ca te pasannākāraṃ kareyyā”ti.
Aggivessana, if these two similes had occurred to you [with reference] to Prince Jayasena, he would have spontaneously acquired confidence in you, and being confident, would have shown his confidence to you.”
“Kuto pana maṃ, bhante, jayasenassa rājakumārassa imā dve upamā paṭibhāyissanti anacchariyā pubbe assutapubbā, seyyathāpi bhagavantan”ti?
“Venerable sir, how could these two similes have occurred to me [with reference] to Prince Jayasena as they occur to the Blessed One, since they are spontaneous and have never been heard before?”
Now, the curious thing is that, at first glance, the syntax suggests the terms are parallel:
dve upamā paṭibhāyeyyuṃ, anacchariyaṃ
dve upamā paṭibhāyissanti anacchariyā
But in fact, as you can see in Ven Bodhi’s translation, they must be construed differently. In the first passage, anacchariya is singular and refers to Jayasena’s “spontaneous” arising of faith. In the second passage it is plural and refers to the “spontaneous” arising of the verses.
It is odd to find these uses together like this, and I can’t help but wonder if there hasn’t been some contamination. Still, odd things happen plenty!
Anyway, so the question would be, is it worthwhile to translate in the same way? We could use something like:
Aggivessana, if these two similes had occurred to you [with reference] to Prince Jayasena, it’d be no miracle if he were to acquire confidence in you, and being confident, would have shown his confidence to you.”
“Venerable sir, how could these two similes have occurred to me [with reference] to Prince Jayasena as they occur to the Blessed One, since they are not miraculous and have never been heard before?”
But I’m inclined to just translate according to the sense in each case:
It wouldn’t be surprising if, had these two similes occurred to you, Prince Jayasena would have gained confidence in you and shown his confidence.”
“But sir, how could these two similes have occurred to me like they did to the Buddha, since they were neither supernaturally inspired, nor learned before in the past?”