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-a Feminine suffix
From Latin -a, from Proto-Italic *, from late Proto-Indo-European *-(e)h2, from Proto-Indo-European *-h2, a suffix indicating collective and abstract animate nouns, from Pre-Proto-Indo-European *-h2, a suffix indicating plurale tantum.
In Pre-Proto-Indo-European, a suffix *-h2 was used to form plurale tantum nouns out of collective nouns. (As a somewhat inaccurate analogy to help the reader understand the function of *-h2 to the ancient Pre-Proto-Indo-European speakers, imagine the collective noun "water" becoming plurale tantum "waters" by way of a plural suffix -s (e.g., "the waters of Babylon" of Psalm 173:1 ESV); the role of the -s is akin to the role of Pre-Proto-Indo-European *-h2). This was attached to the vowel stem *-e-, *-i-, *-u- and *-0- to form *-eh2, *-ih2, *-uh2, and *-h2. The four different endings all to express a single idea of plurality would not last. But now we must discuss the historic Proto-Indo-European method for feminizing words.

In Proto-Indo-European *sor "woman" was added as a suffix to animate nouns (retained in Anatolian languages and partially survived as a fossil in a smattering of non-Anatolian words).
Linguists do not agree as to why *-sor, when used as a feminine marker, was slowly supplanted by the *-(e)h2 collective suffix after the Anatolian languages broke off from the rest of the Indo-European community. Recent theories suggest the suffix played an "individualizing" role. Under these lines of thinking, the collective/abstract suffix *-(e)h2 referred to a subset of larger mass nouns (compare in English plurale tantum "glasses," formed from the mass noun "glass"). Gradually, dependence on *-(e)h2 to refer to subsets of broad categories gave the suffix a new sense of individualization - speakers were depending on the suffix to specify individual items out of larger groups. Once an uncommon suffix, by the late Proto-Indo-European period *-(e)h2 enjoyed heavily use to specify individual animate objects, and so speakers contrasted the new animate ending *-(e)h2 against the old animate ending *-s. *-(e)h2 acquired a new meaning, not one of plurale tantum but a meaning of "the other object."

In compelling support of this theory, there is a word a word that looks to be a relic from the transitional period from collective/abstract suffix to a feminine suffix (the familiar -a in Core Indo-European). *h2u̯idhéu̯eh2- "widow," animate in Anatolian but feminine in other Indo-European languages, looks built off of *h2u̯idhéu̯o- "belonging to the one fatally struck." Thus the original meaning of a widow comes from the time when the suffix played an individualizing role: "the one belonging to the fatally struck." Later, the word simplified to "the bereaved." After the Anatolian branch broke from the Indo-European trunk, the suffix was fully feminized as "the female bereaved." See Luraghi (2011), Tichy (1993), and Melchert (2014b) for a broader discussion.
a (Adverb) "at," "in"
A post-Latin development from the 12th cent., deriving from Latin ad "to," "toward." From Proto-Italic *ad 'id.' From Proto-Indo-European *h2ed 'id.'
Romance: Western Vulgar Latin: Asturian a, Portuguese a, Galician a, Catalan a, French à, Italian a, ad; Eastern Vulgar Latin: Romanian a
Italic: Oscan az "by" (< *ad-s)
Indo-European: Celtic: Welsh â; Germanic: Gothic at, Old Icelandic at, Old English æt (English at)
ab- Prefix indicating "off" or "away"
Not a productive prefix in Spanish, and it appears only as a relic in verbs like abrir. From Latin ab 'id.' From Proto-Italic *ap- 'id.' From Proto-Indo-European *h2epo 'id.' As an allative preposition, the word had a full grade *h2epo (preserved in the prefix ab- above) and a zero grade *h2po (preserved in the prefix po-).
Italic: Umbrian ap-, Paelignian af-, Oscan af-
Indo-European: Germanic: Gothic af, aff-, Old Norse af, Old High German aba, Old Saxon af, English of; Balto-Slavic: Lithuanian po-, Latvian ap "beneath," Old Church Slavonic po, Russian po; Albanian: pa "without;" Messapian: apa "from;" Hellenic: Ancient Greek ἀπό (apó) "far from;" Indo-Iranian: Sanskrit ápa "away," Avestan apa-; Anatolian: Hittite appā "behind"
abajo (Adverb) "down," "below"
First attested c. 1300 but was rarely used until the 15th cent. Formed from Latin a(d)- "at," "toward" and bassus "down" (see a- and bajo respectively).
Also the origin of the surname Abajo, given to Moorish families that lived in the lowlands of Spain.
Romance: Western Vulgar Latin: Asturian (Teberga) abaxu, Portuguese abaixo, Galician abaixo, Catalan abaixo, French à bas
abiertamente (Adverb) "openly"
From abierta and -mente, an adverb-forming suffix.
abierto (Adjective) "open"
12th cent. From Latin apertus 'id.,' from aperire "to open." See abrir.
-able, -ible Prefix indicating ability or inclination.
From Latin -abilis and -ibilis, of identical meanings, both from an a-stem and an i-stem fixed to -bilis "capability." From Proto-Italic *-þli- 'id.' From the Proto-Indo-European instrumental suffix *-dhli-.
ablentar (Verb) "to winnow"
Dialect variant of beldar. From a- and Old Spanish bendlar (a variant bedlar, without the -n-, became the basis for beldar). Formed from 13th cent. Old Spanish bellar via dissimilation. From Latin ventilare "to fan," a verb formed from ventulus "light breeze" (but lit. "little wind"), a diminutive of ventus "wind" (see viento).
Romance: Sardinian: bentulai
abogado (Noun) "lawyer," "advocate," "counsel"
13th cent. From Latin advocatus "aide," "advocate," "witness." From advocare "to advocate" (see abogar).
Historically, abogada meant "interceding saint;" the profession of advocate was exclusive to men prior to the 1960s.
Romance: Western Vulgar Latin: Asturian abogáu, Portuguese advogado, Catalan advocat, Old French avocat, Italian avvocato, Dalmatian abucat; Eastern Vulgar Latin: Romanian a(d)vocat
abogar (Verb) "to plead"
13th cent. From Latin advocare "to summon." From ad- "toward" (see a-) and vocare "to call." Vocare is a verb derived from vox "voice" (see voz).
Romance: Western Vulgar Latin: Asturian abogar, Portuguese advogar, Catalan advocar, French avouer, Italian avvocare