The Onikirimaru (妖刀 鬼切丸 Youtou Onikirimaru) is a katana that was used long ago to cut off Kasen Ibaraki's right arm. The process also sealed Kasen's malevolence into her missing arm. The sword is seemingly currently destroyed or at least broken, as only a fragment of it, kept by Kasen, was shown in Wild and Horned Hermit.
The sword is called 「妖刀 鬼切丸」 by Kasen Ibaraki. 妖刀 (Youtou) literally means "mystical blade", or "bewitched blade". The second part, 鬼切丸 (Onikirimaru), means "Oni-cutter" and uses the suffix 丸 (maru), which is commonly seen in names.
The sword seems to be based on the mythical sword used by Watanabe no Tsuna, a legendary warrior that according to myths cut off Ibaraki-douji's arm at Rashōmon after it escaped the oni extermination on Mt. Ooe. Tsuna's sword had many names, each coming from a particular event involving it; one of these names was 鬼切 (Onikiri), and was granted to the sword after it was used to cut off Ibaraki-douji's arm.
The full shape of Onikirimaru as shown in Wild and Horned Hermit