I have revised this little note in 2023, so the comments may reflect an earlier version.
In MN 40, we have a description of a bad mendicant. Their bad qualities and behavior are said to be like a deadly dagger that they keep stashed away inside their outer robe (saṇghāti). The translation by Ven Bodhi doesn’t convey the point of the simile, though it was caught by the earlier translations by Horner and Chalmers.
The description of the weapon is unique and difficult. With word-by-word rendering
matajaṃ nāma āvudhajātaṃ ubhatodhāraṃ pītanisitaṃ
mataja-named—kind of weapon—double-edged—(yellow)-whetted
The commentary gives a long explanation for mataja, which literally means “deadborn”. First you mix up iron filings with meat, then feed it to a heron. When the heron has had enough it dies or is killed, and the iron filings are recovered from its belly. Skilled metalworkers forge a weapon from them. Perhaps the digestive fluids were felt to chemically harden the metal?
Dhārā here means edge, in a sense apparently unattested elsewhere in the EBTs. (But see Brahmali’s comment below.)
Nisita means “sharpened” or “whetted”.
Pīta is problematic. It means either “having drunk” or “yellow”, neither of which makes obvious sense. The commentary takes it in the former sense, saying that the blade has been whetted on a stone soaked in water. This sense has been accepted by previous translators.
However, the Sanskrit dictionaries give another sense of pīta: orpiment or “yellow arsenic”.
This was widely used in metallurgy, especially for hardening bronzes for weaponry. This was invented in Iran in the 4th millenium BCE, from where it spread to India. The garlic-smelling fumes of toxic arsenic were so characteristic of sword-smithies that they are, so it seems, the cause of the Greek god Hephaestus’ lameness. Alone of all the gods, he suffered a physical defect, which was probably inferred from the common sight of metalworkers suffering lameness through muscle atrophy due to long-term exposure to arsenic and lead.
Whether this sense is meant is uncertain. But there is another nuance to this. Arthaśāstra 2.14.48 describes how metals may be rubbed with yellow arsenic (here = haritāla) to disguise them. The abrasive yellow qualities of orpiment were used for polishing mirrors. And apparently this was exploited as a trick by merchants to pass off inferior metals.
This fits the context well, as the idea is of a weapon that is disguised by a “yellow” gloss, like a bad monk wrapped in their “yellow” robes.
This brings us back to nisita. As well as “whetted”, the Sanskrit dictionaries give niśita in the sense of “strengthened, prepared” and even “iron, steel”. These meanings probably overlap, used for a hardened and whetted blade. But it also has the sense of “excited, eager”, which by good luck we can capture with “keen”.
Perhaps we could translate the phrase as:
matajaṃ nāma āvudhajātaṃ ubhatodhāraṃ pītanisitaṃ
the kind of weapon called ‘deadborn’—double-edged, whetted with yellow arsenic