"a timber set upright," from Old English post "pillar, doorpost," and Old French post "post, upright beam," both from Latin postis "door, post, doorpost," perhaps from por- "forth" (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 "ridepole," Lithuanian pirštas, Old Church Slavonic pristu "finger" (PIE *por-st-i-).