be somebody's middle name

be (one's) middle name

To be a trait that one strongly possesses or readily displays, or a thing that one considers to be closely associated with oneself. Often used humorously or ironically. Are we going to have fun? You bet, kid—"adventure" is my middle name! You don't need to worry about Donna—"safety" is her middle name.
See also: middle, name

be somebody’s middle ˈname

(informal) used to say that somebody has a lot of a particular quality: ‘Patience’ is my middle name!
See also: middle, name