If you finish enough Abstergo challenges - as below - you will unlock the named cheats. They 
can be found in the Cheats option in the Abstergo Challenges menu. 
Note: You cannot save the game while these are in use.

Cheats                      How to unlock
------------------------------------------------------------
Armed to the Teeth        - Complete 90 Abstergo Challenges.
Arrr Matey!               - Complete 20 Abstergo Challenges.
Celestial Navigation      - Complete 10 Abstergo Challenges.
Dead Men Tell No Tales    - Complete 70 Abstergo Challenges.
Deceased Crew             - Complete all 100 Abstergo Challenges.
Loaded to the Gunwhale    - Complete 50 Abstergo Challenges.
No Quarter                - Complete 30 Abstergo Challenges.
Poseidon's Will           - Complete 60 Abstergo Challenges.
Scourge of the Seven Seas - Complete 40 Abstergo Challenges.
Shiver Me Timbers         - Complete 80 Abstergo Challenges.