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 earn Achievements or 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.