(from comprehensive rulebook) 502.38. Bushido
502.38a Bushido is a triggered ability. “Bushido N” means “Whenever this creature blocks or becomes blocked, it gets +N/+N until end of turn.” (See rule 309, “Declare Blockers Step.”)
502.38b If a creature has multiple instances of bushido, each triggers separately.
This ability has only been seen in samurai from the Kamigawa Block (and Greater Morphling from Unhinged) so far, obviously because of it's name, which is Japanese for "way of the warrior" and the samurai honor code.