Like old times, yar! is one of 28 Achievements in The Respawnables.
The description says Kill 50 enemies with the Blunderbuss
This achievement is earned after killing 50 enemies with the blunderbuss.
Note that only enemies in the Mission Mode count as enemies.
Enemies in Multiplayer do not work for this Achievement.
The reward for this achievement are 10 Score Points (when achieved during a match).
Tips & Tricks
That one is pretty easy actually, since the Blunderbuss is a strong and relative cheap weapon.
If you still have problems with getting this Achievement, check this page for detailed strategies in playing with Blunderbuss.