mid-15c., "having or intended to show faith in and reverence for the Supreme Being," from Latin pius "dutiful, devout, conscientious, religious; faithful to kindred; inspired by friendship, prompted by natural affections," perhaps [de Vaan, Klein] related to Latin purus "pure, clean," via a PIE *pu-io- "purifying" (see pure), but the exact development is disputed.
The classical Roman sense of "having or exhibiting due respect and affection for parents and others to whom such is due" is attested in English from 1620s. In the religious sense, sometimes denoting practice under pretense of religion or for good ends (1630s) and in this sense often coupled with fraud (n.). Related: Piously; piousness.
<a href="https://www.etymonline.com/word/Pius">Etymology of Pius by etymonline</a>
Harper, D. (n.d.). Etymology of Pius. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Pius