This base type goes back to the Latin etymon
bassus 'low' (cf.
REW 978, under
bassus). In Classical Latin it is really only documented as an addition to family names in the sense of 'the fat one' (cf.
Georges 1, 793 under
bassus). In most glosses, when used as an adjective, it bears the pejorative connotation 'fat, thick, stocky', which is also the original meaning of the word. It thus does not mean low in the sense of 'low-lying' in this case (cf.
FEW 1,
275 under bassus). In French, it became the adjective
bas, which can change its meaning depending on whether it precedes or follows the noun. If one wants to attribute an external or geographical quality to a noun, it follows the noun. If, however, the speaker wishes to express a subjective evaluation, it is placed before the noun, which is thereby devalued (cf.
TLFi, under
bas). The
fra. babeurre 'buttermilk' is a composition of two base types:
bassus and
butyrum 'butter' (cf.
TLFi, under
babeurre; see also
babeurre (m.) (roa.)). The conjunction with
fra. bas expresses that buttermilk was considered only as a waste product. It is generated during the production of butter and contains mostly water. The farmers in the Alps consumed it only very rarely; it was mainly used for the production of whey cheese or fed to the pigs.
Georges, Heinrich (1913-1918): Ausführliches lateinisch-deutsches Handwörterbuch. Aus den Quellen zusammengetragen und mit besonderer Bezugnahme auf Synonymik und Antiquitäten unter Berücksichtigung der besten Hilfsmittel ausgearbeitet, Hannover, Hahnsche Buchhandlung
LinkWartburg, Walter (1922-1967): Französisches etymologisches Wörterbuch. Eine Darstellung des galloromanischen Sprachschatzes , Basel, vol. 20, Zbinden
Link