What is inattentiveness? It is a discriminating awareness which is simultaneous with and on the same level as the emotions and thereby is made inattentive regarding actions by body, speech, and mind. It has the function of providing a basis for falling from one's level of being.
It is an emotionally tainted discriminatingh awareness which lacks watchfulness with regard to the activities of body, speech, and mind and is not associated with carefulness.

That this inattentiveness becomes the foundation for falling from one's level of being is as the Bodhicaryavatara states,

A person who is learned and has trust
But does not apply himself diligently
Will be sullied by falling from his status
Because the defect of not being watchful has clung to him.