Adjective

(en adjective)

Not secure.

Not comfortable or confident in oneself or in certain situations.

*{{quote-book, year=1963, author=(Margery Allingham)

, title=(The China Governess) , chapter=1 citation , passage=The half-dozen pieces […] were painted white and carved with festoons of flowers, birds and cupids. […]  The bed was the most extravagant piece.  Its graceful cane halftester rose high towards the cornice and was so festooned in carved white wood that the effect was positively insecure , as if the great couch were trimmed with icing sugar.}}

He's a nice guy and all, but seems to be rather insecure around other people.

Adjective

(en adjective)

(archaic): Lacking confidence in others; distrustful.

Lacking confidence in one's self; distrustful of one's own powers; not self-reliant; timid; modest; bashful; characterized by modest reserve.

*

Having therefore—but hold, as we are diffident of our own abilities, let us here invite a superior power to our assistance.

* {{quote-book

, year=1960 , author= , title=(Jeeves in the Offing) , section=chapter VIII , passage=At an early point in these exchanges I had started to sidle to the door, and I now sidled through it, rather like a diffident crab on some sandy beach trying to avoid the attentions of a child with a spade.}}