Less than normal.
denormal
A person whose abilities are less than normal.
That part of the axis of a curved line which is intercepted between the ordinate and the normal.
Barely noticeable, not obvious, indistinct.
Tenuous; rarefied; of low density or thin consistency.
Cleverly contrived.
Cunning, skillful.
Insidious.
Hard to grasp; not obvious or easily understood.