There doesn't seem to be an "official" team motto (Reference.com claims that it is "Youth, Courage, Greatness," but I cannot confirm that information).
However, the original Manchester United logo was a coat of arms with the motto 'Concilio et Labore' - which means "Council and Labor" or more like "Wisdom and Effort" in English, which is something that could definitely be interpreted as a club motto.
They do have an official vision statement, which is as follows:
Our vision is to be the best football club in the world both on and off the pitch, and to help us
achieve this we live by the UNITED values:
The Manchester United Foundation also has an official goal:
To educate, motivate and inspire future generations to build better communities for all.
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Manchester United's nickname is the 'Red Devils' hence the devil present in the logo.
If you mean the old Shirt Sponsor AIG; it stands for American International Group.