What is lack of sense of propriety? It is not restraining oneself by taking others as the norm. It is an emotional event associated with passion-lust, aversion-hatred, and bewilderment-erring. It aids the basic emotions and the proximate emotions. 

-Abhidharmasamuccaya

It is a subjective tendency not to curb (restrain) evil by taking others as the norm. Again, concerning this, when one is tempted to do evil (and restrains from doing so) and thinks, "It is not proper to be frowned upon by others who are worthy of respect, such as teachers and the Gods who look into the mind of others" -- one thus avoids evil by taking others as the norm. This is decorum. The opposite of this is the lack of the sense of propriety. The Ratnamala states,

Shamelessness and lack of a sense of propriety
Means not to exercise restraint in view of oneself and others.

The Bodhisattvabhumi states,

Self-respect is a Bodhisattva's self-restraint in the knowledge that any indulgences and impropriety is not his way. Decorum is this restraint and fear and respect of others.

Both the lack of self-respect and the lack of the sense of propriety aid all of the emotions and are the causes of all evil. If one does not want to refrain from evil, one cannot protect oneself against it. Therefore, both the earlier and later Abhidharma works state that the lack of self-respect and the lack of propriety are said to be on the same level as and correspond to all unhealthy attitudes. Although more could be said, this will suffice.