Bonding Knife
A Bonding Knife is essentially a worthless piece of wargear used by the Fish Lips. Due to the fact that Tau are so shit in close combat that they will get their ass handed to them by a fucking Grot. These knives are basically there just for show, like those plastic katanas you buy from Walmart, easily the worse piece of close combat weapon in the entirety of 40k.
So What is Their Purpose Really?
The Bonding Knife, because of how pointless they are in combat, serves more as a ceremonial knife that serves as part of the Tau ritual of the Ta'lissera, in which a group is bonded together through ritualistic cutting of their flesh. Who knew the Weebs were into Imperial Fist-tier of masochism eh? Following their ritual bonding, the knife is then carried into battle by the team leader, although it is not designed for use in actual combat, and serves as a reminder of the sacred bond the group shares. Fire Warriors who serve as part of a Battlesuit team may also undergo the process, although instead of carrying the actual knife into battle the team leader may instead paint a knife design onto his armour.
Ta'lissera
The Ta'lissera is a Tau ceremony which has been translated by Imperial xenolexicons as "communion" or "marriage." Tau who take part in the ceremony pledge support and community to one another and may address each other by their individual names. The bonds that these Tau swear to each other are much admired and seen as embodying the best aspects of the Greater Good. While the bond may be performed by any group of Tau, it is most commonly Fire Caste warriors or Earth Caste work teams who undergo this ultimate expression of respect for one another.
Tl;dr
These things are for Roleplay, no really! It is as every bit as nonsensical, contrive and pointless as its existence. It also looks like a wakizashi short sword. Ya know, because the cunt-noses just aren't Weeb enough. Always trust the Tau in creating things that somehow, have less testosterone than the Eldar.