c.1200, from Old French piler "pillar, column, pier" (12c., Modern French pilier) and directly from Medieval Latin pilare, from Latin pila "pillar, stone barrier." Figurative sense of "prop or support of an institution or community" is first recorded early 14c. Phrase pillar to post is c.1600, originally of tennis, exact meaning obscure.
pillar pil·lar (pĭl'ər)
A structure or part that provides support and resembles a column or pillar.
From one place or thing to another in rapid succession: “Abernathy couldn't stick to one project and was always dashing from pillar to post.”