Upright means standing straight up. An upright post, sit upright, keep it upright.
It describes both position and posture. An upright piano is the tall kind, as against a grand, and an upright is also a vertical structural post. Of a person, upright means honest and morally straight: an upright citizen — which is exactly the same metaphor as straight and standing. Note bolt upright, meaning suddenly and completely straight; and that the word has meant both the physical and the moral thing since Old English.
1adjectiveuprightStanding straight up.
“Please return your seat to the upright position for landing.”
2adjectivehonestHonest and morally straight.
“He was an upright man in a thoroughly bent trade.”
upright is a vertical position or moral quality.
Old English upriht is up plus right, where right meant straight rather than correct. The moral sense was there from the start, because straightness has been a metaphor for honesty for as long as English has existed.
Proto-Indo-European
*h₃reǵ-
Proto-Germanic
*upprehtaz
Old English
upriht — upright; erect
Middle English
upright
English, today
upright
From Middle English upright, uppryght, upriht, from Old English upriht (“upright; erect”), from Proto-Germanic upprehtaz, equivalent to up + right. Cognate with stq apgjucht (“upright”), West Frisian oprjocht (“upright”), Dutch oprecht (“upright”), German Low German uprecht (“upright”), German aufrecht (“upright”), Swedish upprätt (“upright”), Icelandic uppréttur (“upright”).
Etymology from Wiktionary, CC BY-SA