Reconstruction:Proto-Indo-European/h₁epi

Would be locative case of *h₁ep-, which could mean "back", with the "on" sense referring to pack animals or riding.

*h₁epi

1. on, at, near

• *h₁op-s
  • Italic: [Term?]:
    • Latin: obs
• *h₁op-i-s
  • Hellenic: [Term?]:
    • Ancient Greek: ὄπισθεν (ópisthen), ὀπίσω (opísō), ἀνόπιν (anópin), κατόπιν (katópin), μετόπιν (metópin).

• Albanian:
  • Albanian: mbi (Tosk)
  • Albanian: mby, Albanian: my (Gheg)
• Armenian:
  • Old Armenian: եւ (ew)
    • Armenian: եւ (ew)
• Balto-Slavic:
  • Baltic:
    • Latvian: piedurkne
    • Lithuanian: ap-
• Celtic: *eɸirom
  • Old Irish: íar
• Germanic: *bi (see there for further descendants)
• Germanic: *ēbanþs (see there for further descendants)
• Hellenic:
  • Mycenean Greek: [script needed] (o-pi)
    • Ancient Greek: ἐπί (epí)
      • Greek: επί (epí)
• Indo-Iranian: *Hápi (see there for further descendants)
• Italic:
  • Latin: ob, ap(ud)
  • Oscan: 𐌞𐌐 (úp), op[1]

• Sihler, Andrew L. (1995) New Comparative Grammar of Greek and Latin, Oxford, New York: Oxford University Press, →ISBN
• Pokorny, Julius (1959) Indogermanisches etymologisches Wörterbuch [Indo-European Etymological Dictionary] (in German), volume III, Bern, München: Francke Verlag, page 841