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 evidently missed 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 shows it thought it was based on maraṇa. I don’t have a problem with this; maybe we could say, “the kind of weapon called ‘death-dealer’” or something.
Dhārā here means edge, in a sense apparently unattested elsewhere in the EBTs.
Nisita has been rendered by previous translators as “sharpened” or “whetted”, but see below.
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.
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.
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 ‘death-dealer’, double-edged, hardened and keen