Shameless, a. — Webster enters it under “shameless”
Etymology: [AS. scamleás.]
1. Destitute of shame; wanting modesty; brazen-faced; insensible to disgrace. "Such shameless bards we have." Pope. Shame enough to shame thee, wert thou not shameless. Shak.
2. Indicating want of modesty, or sensibility to disgrace; indecent; as, a shameless picture or poem.
Syn. -- Impudent; unblushing; audacious; immodest; indecent; indelicate. -- Shame"less*ly, adv. -- Shame"less*ness, n.
Shades of meaning
Fernald has no entry of its own for shamelessness; he lists it under effrontery.
Where he writes about it:
Audacity, in the sense here considered, is a reckless defiance of law, decency, public opinion, or personal rights, claims, or views, approaching the meaning of impudence or shamelessness, but always carrying the thought of the personal risk that one disregards in such defiance; the merely impudent or shameless person may take no thought of consequences; the audacious person recognizes and recklessly braves them. — from effrontery
Effrontery (L. effrons, barefaced, shameless) adds to audacity and hardihood the special element of defiance of considerations of propriety, duty, and respect for others, yet not to the extent implied in impudence or shamelessness. — from effrontery
Impudence disregards what is due to superiors; shamelessness defies decency. — from effrontery