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.

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