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Equus:

(Classical) IPA(key): /ˈe.kʷus/, [ˈɛ.kʷʊs]


Aequus:

(Classical) IPA(key): /ˈae̯.kʷus/, [ˈae̯.kʷʊs]

Is this similarity coincidental or do they have a common origin? Are there any specific reasons why the two words are so similar-sounding - some cultural reason, myth, belief, tradition of respect towards horses or the like?

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According to http://starling.rinet.ru, they come from different Proto Indo-European roots. The database is based in the Indo-European Etymological Dictionary project.

  • aequus would come from *aikʷ-, meaning flat and also present in Baltic languages such as Lithuanian.
  • equus would come from *ek'w-, meaning horse, properly. Hippos comes from the same root.

I'm no Proto-IE expert, but apparently these are regular transformations, and it is nothing but a coincidence (or, at most, an example of convergent evolution).

Caveat: since Proto-IE was never written, all we have is a theoretical reconstruction, mostly agreed by scholars (and thus reliable enough for modern-day purposes). That is why roots always start with an asterisk (*): they aren't properly attested. Maybe if we travelled back in time we'd be surprised.

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  • 2
    Not sure where you got that fact about caballo: what about Latin caballus? Also, out of curiosity, it baffles me (with no formal linguistics training) how hippos could come from *ek'w, as you mention. Any thoughts?
    – brianpck
    May 18 '16 at 15:21
  • 1
    @brianpck Actually my memory betrayed me. Shall I delete the comment? However there is the fact that PIE rooting does not rule out a previous common origin. Regarding the development of hippos, sound changes may be surprising. See the last one in this list (merging sequences of velar + ...)
    – Rafael
    May 18 '16 at 18:36
  • 1
    The PIE forms do rule out a common origin, since they don't have a single sound in common (*kʷ is not the same as *k'w).
    – TKR
    May 22 '16 at 7:02
  • 3
    @TKR. aequus does not have any generally accepted cognates outside of Italic (the Lithuanian parallel suggested in the “Starling” link is questionable). But this means that we do not know whether it goes back to a form with kʷ or with ḱw, as these would have an identical outcome in Latin. The “horse” word definitely has ḱw (cf Sanskrit aśva-).
    – fdb
    May 29 '16 at 10:25
  • 2
    @fdb Good point. Still, even if aequus reflects PIE ḱw, I don't see any obvious PIE morphological process that could relate its ancestor to that of *equus.
    – TKR
    May 29 '16 at 16:44

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