Politeness is a tacit agreement that people’s miserable defects, whether moral or intellectual, shall on either side be ignored and not made the subject of reproach; and since these defects are thus rendered somewhat less obtrusive, the result is mutually advantageous.

[Sie ist eine stillschweigende Übereinkunft, gegenseitig die moralisch und intellektuell elende Beschaffenheit von einander zu ignoriren und sie sich nicht vorzurücken; – wodurch diese, zu beiderseitigem Vorteil, etwas weniger leicht zutage kommt.]

Arthur Schopenhauer (1788-1860) German philosopher

Parerga and Paralipomena, Vol. 1, “Aphorisms on the Wisdom of Life [Aphorismen zur Lebensweisheit],” ch. 5 “Counsels and Maxims [Paränesen und Maximen],” § 3.36 (1851) [tr. Saunders (1890)]
    (Source)

Source (German). Alternate translation:

Politeness is a tacit agreement that we shall mutually ignore and refrain from reproaching one another's miserable defects, both moral and intellectual. In this way, they do not so readily come to light, to the advantage of both sides.
[tr. Payne (1974)]