Impervious (adj): not allowing anything to pass through, impenetrable

A raincoat, if it is any good, is impervious to water, It is made of an impervious material

Impetuous (adj) impulsive, extremely impatient

She was so impetuous that she never took more than a few seconds to make up her mind.

Implement (v) to carry on

Impotent (adj) powerless, helpless, unable to perform

We felt impotent in the face of their overpowering opposition to our plan

Impugn (v): to attack, especially to attack the truth or integrity of something.

 

Inane (adj) silly, senseless

Their plan was inane

Inaugurate (v) to begin officially, to induct formally into office