from Löpelmann's dictionary of the Basque in France
pede: abl. "(at the) foot" Latin
pé "foot, foundation, base" Galician,
Portuguese
pé de "beside" Galician
ao pé da lettra "literally" Portuguese
be- pref. "(going) inside or below" Basque
pe "underside, ground floor,
the ground" Basque
-be, -bi,
-pe, -pia suff. "under" Basque
-pean, -pian
(inessive of pe) "under" (after indef. nouns
or poss. gen.) Basque
pera adlative "going under, towards" Basque
petik "from below, hidden,
going below and through" Basque
peka adv. "below, uner the ground, hidden" Basque
peko "lower, subordinate,
lower in rank" Basque
Löpelmann thinks Basque borrowed the root from Latin or Romance,
which it then turned into pre- and suffixes. The free word <pe> might
as well have been "reconstituted" from the suffix (-pe), as Miguel
proposed for <tegi> "shed". Semantically it doesn't seem right that
it should have been borrowed from a word with the primary
meaning "foot". In which case it might be ancient in Basque and
borrowed from elsewhere. Note Russian <pod> "under".
I liked what I found here, since I earlier proposed that derivatives
of <ap-> "water" (itself a loanword) entered IE as postpositions,
constructed as {<noun>-gen.suff. 'ap-'-local-case-suffix} (local
case: locative, allative, ablative etc.). It seems Basque used
the "foot", or rather "surface below", word before I even came up
with the idea.
Torsten