Bigan is a variation of the Shigan.
Kaku uses his nose here in the giraffe-man form, which he then smashes at his opponents at high speed instead of a finger as is usual with the Shigan. The nose has so much power at this speed that it can shoot a square hole in a stone. So it works exactly like a finger in the Shigan.
With this attack name Kaku further shows his arrogance towards Zoro, which the latter notices throughout the fight, because the pun sounds very self-indulgent. After using the attack several times, Zoro said that Kaku’s arrogance gets on his nerves a lot, as he understands the pun. This is further increased by the use of the Kyoku Bigan: Kilimanjaro, an amplified form of the Bigan.
Bigan (美顔) means “pretty face”.
In kanji it means “nose gun”.