To acquire this item, you must trade a Buccaneer's Knife to the ??? near the area where Brigandish Blade is spawned. Trade the Buc's Knife after each BB kill and good luck :).
Just got this last night. Meandered over to the ??? maybe 2-3 seconds after it died. About the time it took to run over to the ??? from his pop location and casually trade it. (I killed it on war with the knife)
So at the very least you don't need to panic and spam it with some /item macro. I got it on the first or second try.
Just did some testing outside of whitegate. Added effect did 0 to colibris and 23 to spiders, both had very low proc rates of 1-3 times per mob.
Just got this knife yesterday. I didn't even know about the trade so my LS had to tell me what to do after we killed NM. Needless to say, it was a LONG time after NM death before I traded, so I don't think it has to be very quick at all. I took at least two full minutes before I walked over to trade.
you need to make a macro to do it, and spam the macro the instant BB dies. you (likely) have less than 1 second to trade it after he dies. be right in front of of the ??? if you can. this is NOT the ??? used to spawn BB, but the one near the tunnels at the south end of the room.
make the macro:
/targetnpc
/item "Buccaneer's Knife" <t>
if you are the one using the bucc knife to kill him, just make another macro to hit first to unequip it.
/equip main "Some other weapon"
it worked for me.