Apadāna is a difficult word. It is famously used for stories of past lives, a usage that apparently originated with the Mahāpadānasutta at DN 14. However it is occasionally found in other places in the canon, which appear to reflect an earlier sense out of which the idea of “legends of past lives” grew.
The problem is that the few references are themselves obscure and it’s somewhat difficult to pin down the common thread.
Cone’s DoP gives the senses of “cutting, reaping, harvest”, and then “a person’s reaping the result of actions”.
The sense of “harvest” is from DN 27:17.23, where we find:
(the rice) did not grow back after reaping, the apadāna showed, and the rice stood in clumps.
Then in DN 24:2.7.3 a bad monk was was criticized because while:
living on the apadāna of the Holy One, enjoying the leftovers of the Holy One, you presume to attack the Realized One
Here “harvest” also works well.
Elsewhere, however, the commentary glosses apadāna as “conduct”, and while Cone sort-of explains it as “result or harvest of conduct”, it doesn’t really seem as if the commentary means that, or indeed if it is supported by the contexts.
A list of bad deeds in MN 15 includes,
When accused, they are unable to account for the apadāna
While at AN 3.2 we find the Buddha apparently quoting an aphorism, whose translation is quite tricky. The sense of the sutta is that a fool or an astute person is known by their deeds, and:
apadānasobhanī paññā
Which Bhikkhu Bodhi translates as “wisdom shines in its manifestation”. Again the commentary explains as “conduct”.
Now, it seems to me that the explanations have built on the most obvious usage (“harvest”) and then try to infer to the less apparent cases—normally a good procedure! But when reviewing all these contexts, what stood out to me was there is a common aspect to these which is not really prominent in the explanations.
If we return to DN 27, the text is talking about an act that leaves traces behind. We assume that is a harvest because of context, but it need not be. Likewise in DN 24, the primary sense is that someone is enjoying the traces left behind by the Buddha. This makes sense also in AN 3.2, which is also about how traces are made known, in this case, how one’s level of wisdom leaves traces in one’s conduct. Finally, this fits the sense of MN 15, in the sense that the monks confront a bad monk with the “evidence” of his conduct which he is unable to account for.
Thus I propose that the primary sense of apadāna is “evidence”, “traces or marks or things left behind”.
DN 27: (the rice) did not grow back after reaping, leaving traces showing, and the rice stood in clumps.
DN 24: living on the things left behind by the Holy One, enjoying the leftovers of the Holy One, you presume to attack the Realized One
MN 15: When accused, they are unable to account for the evidence
AN 3.2: Wisdom shows in the traces it leaves behind
Thus the connection with “harvest” is incidental, and the connection with “stories of past lives” becomes clear. It is about how things done in the past leaves “traces” that affect the present.
As for Mahāpadānasutta, we might translate as "The Great Discourse on Traces left Behind”.