O.Henry is writing about a humorist, who so annoyed his family by haunting them to pick up material for jokes that he became a "pariah". I knew what it meant, but didn't know the origin, so I looked it up:
Webster's 1913 Dictionary

Definition: \Pa"ri*ah\, n. [From Tamil paraiyan, pl. paraiyar, one of
the low caste, fr. parai a large drum, because they beat the
drums at certain festivals.]
1. One of an aboriginal people of Southern India, regarded by
the four castes of the Hindoos as of very low grade. They
are usually the serfs of the Sudra agriculturalists. See
{Caste}. --Balfour (Cyc. of India).

2. An outcast; one despised by society.

{Pariah dog} (Zo["o]l.), a mongrel race of half-wild dogs
which act as scavengers in Oriental cities.

{Pariah kite} (Zo["o]l.), a species of kite ({Milvus
govinda}) which acts as a scavenger in India.