signpost n.

early 13c., signe, "gesture or motion of the hand," especially one meant to express thought or convey an idea, from Old French signe "sign, mark," from Latin signum "identifying mark, token, indication, symbol; proof; military standard, ensign; a signal, an omen; sign in the heavens, constellation."

According to Watkins, literally "standard that one follows," from PIE *sekw-no-, from root *sekw- (1) "to follow." De Vaan has it from PIE *sekh-no- "cut," from PIE root *sek- "to cut" He writes: "The etymological appurtenance to seco 'to cut' implies a semantic shift of *sek-no- 'what is cut out', 'carved out' > 'sign'." But he also also compares Hebrew sakkin, Aramaic sakkin "slaughtering-knife," and mentions a theory that "both words are probably borrowed from an unknown third source."

It has ousted native token. By c. 1300 as "an indication of some coming event." The meaning "a visible mark or device having some special meaning" is recorded from late 13c.; that of "miraculous manifestation, a miracle demonstrating divine power" is from c. 1300. In reference to one of the 12 divisions of the zodiac, from mid-14c.

The sense of "inscribed board with a characteristic device attached to the front of an inn, shop, etc.," to distinguish it from others is recorded from mid-15c. The meaning "indicator, token or signal of some condition" (late 13c.) is behind sign of the times (1520s). The meaning "conventional mark or symbol in place of words" (in music, mathematics, etc., as in plus sign) is by 1550s. In some uses, the word probably is a shortening of ensign. 

"a timber of considerable size set upright," from Old English post "pillar, doorpost," and from Old French post "post, upright beam," both from Latin postis "door, post, doorpost," in Medieval Latin "a beam, rod, pole," which is perhaps from Vulgar Latin *por- "forth," a variant of pro- (see pro-) + stare "to stand" (from PIE root *sta- "to stand, make or be firm").

Similar compounds are Sanskrit prstham "back, roof, peak," Avestan parshti "back," Greek pastas "porch in front of a house, colonnade," Middle High German virst "ridgepole," Lithuanian pirštas, Old Church Slavonic pristu "finger" (PIE *por-st-i-).

Later also of metal. As a type of hardness, lifelessness, deafness by early 15c.