This is latin word .used in law . It is also called legal maxim
here actus means act
dei means god
nemini means kind of no
so the basic meaning is "no one resposible when something (wrong) is done by god (nature)
eg extreame storm , or big rainfall , tsunami etc
and because of this some one god injured then no one will go to jail.
Eg suppose extreme extreme strom came and your car goes in the air and drop on somebody's house . And his hous is broken then he can't sue u . He can sue u but he can't will that law suit . The judge will say it was act of god or actus dei nemini facit injuriam