c. 1200, piler, "a column or columnar mass, narrow in proportion to height, either weight-bearing or free-standing," from Old French piler "pillar, column, pier" (12c., Modern French pilier) and directly from Medieval Latin pilare, from Latin pila "pillar, stone barrier," a word of unknown etymology. The figurative sense of "prop or support of an institution or community" is recorded from early 14c. Related: Pillared.
In medieval architecture often made so as to give the appearance of several shafts around a central core; "by architects often distinguished from column, inasmuch as it may be of any shape in section, and is not subordinated to the rules of classic architecture" [Century Dictionary].
Phrase pillar to post "from one thing to another without apparent or definite purpose" is attested from c. 1600, late 15c. as post to pillar, mid-15c. as pillar and post; but the exact meaning is obscure. Earliest reference seem to allude to tennis, but post and pillar is recorded as the name of a game of some sort c. 1450. The theory that the expression is from pillar as the raised ground at the center of a manège ring around which a horse turns is unlikely because that sense seem to date only to 18c.
The Pillars of Hercules are the two hills on opposite sides of the Straits of Gibraltar, Abyla in Africa and Calpe in Europe, said to have been torn asunder by Hercules.