In various languages, it has been used by millions of Scouts around the world since 1907.
Of course, it is no coincidence that this motto can be shortened to B. P. and Robert Baden-Powell, the founder of the movement, used to shorten his surname into B.-P.
In the third part of Scouting for Boys dated February 12, 1908, Robert Baden-Powell explains the meaning of the phrase:
The Scout Motto is: BE PREPARED which means you are always in a state of readiness in mind and body to do your DUTY.
Be Prepared in Mind by having disciplined yourself to be obedient to every order, and also by having thought out beforehand any accident or situation that might occur, so that you know the right thing to do at the right moment, and are willing to do it.
Be Prepared in Body by making yourself strong and active and able to do the right thing at the right moment, and do it.