The Unlocking Charm (Alohomora), also known as the Unlocking Spell or the Thief's Friend was a charm that unlocked objects such as doors or windows. It was also able to open doors locked by the Locking Spell (Colloportus), and as such, acted as its counter-charm. The Anti-Alohomora Charm could be used to prevent this charm from working on a locked target. 

Surprisingly there is no etymology but we shall get into the history!

HISTORY: The Unlocking Charm had its origins in Africa, whereupon it was called the Thief's Friend. The first known person to have mastered the charm was an Ancient African sorcerer, who taught it to thief Eldon Elsrickle during the latter's trip to the continent in the 17th century. Upon returning to his native Britain, Elsrickle used it to terrorize London, stealing from homes of Muggles and wizards alike, until Blagdon Blay invented the Anti-Alohomora Charm. Before he died, Elsrickle gave knowledge of the Unlocking Charm in exchange for his freedom on the infrequent occasions he broke into a magical household, resulting in the spell being widespread in the country.

Onto the movement and pronounciation!

follow the movements whilst repeating these words. (ah-LOH-ho-MOR-ah)

Goodluck!