hachigane- The metallic "bowl" area of the traditional Japanese helmet (kabuto) that covers the wearer's forehead. Literally translated, the term means "pot-metal."
kodachi- I didn't explain it in Vol. 3 but there isn't much to explain in the first place. It's just a relatively small sword and can sometimes be "stuck" inside an obi of a kimono (so in that sense it's not that different from a wakizashi).
Mugenjin- I might have translated this in a previous episode but just forgetten about it. But literally it means "infinite blade."
Kou, Otsu- In this usage, it merely is the old Japanese way of saying "this and that," "A and B," etc. (bascially two sides of a pair.
Daijmoni (no okuribi)- "The letter LARGE." Okuribi, as translated in the subtitles is the bonfire lit to send off the spirits of the dead back into the afterlife in the final phase of the Obon season (mid-July in modern calendars, mid-August in the old/Chinese calendars). Kyoto has a large bonfire in the shape of the kanji "dai (forge)" on the side of a mountain.
