Etymology
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filial (adj.)
late 14c., from Late Latin filialis "of a son or daughter," from Latin filius "son," filia "daughter," possibly from a suffixed form of PIE root *bheue- "to be, exist, grow" (see be), but Watkins finds it "more likely" assimilated from *felios, originally "a suckling," a suffixed form of PIE root *dhe(i)- "to suck, suckle."
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filioque 
Latin, "and from the son," from ablative of filius "son" (see filial). "Clause in Nicene Creed which separates Eastern Church from Western" [Weekley].
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affiliate (v.)
1761, "bring into close association," from Latin affiliatus, past participle of affiliare "to adopt a son," from ad "to" (see ad-) + filius "son" (see filial). Outside legal use, always figurative. Related: Affiliated; affiliating.
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filicide (n.)
1660s, "action of killing a son or daughter," from Latin filius/filia "son/daughter" (see filial) + -cide "a killing." Meaning "one who kills a son or daughter" is from 1823. Related: Filicidal.
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fitz (n.)
Anglo-French fitz, from Old French fils, from Latin filius "son of" (see filial); used regularly in official rolls and hence the first element of many modern surnames; in later times used of illegitimate issue of royalty.
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filiation (n.)

1520s, "process of becoming, or state of being, a son," from French filiation, from Medieval Latin filiationem (nominative filiatio), noun of action from past-participle stem of filiare "to have a child," from Latin filius/filia "son/daughter" (see filial). As "relationship of a son or daughter to a parent" (correlative of paternity) from 1794.

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affiliation (n.)

1751, "adoption," from French affiliation, from Medieval Latin affiliationem (nominative affiliatio), noun of action from past-participle stem of Latin affiliare "to adopt as a son," from ad "to" (see ad-) + filius "son" (see filial).

The figurative sense of "adoption by a society," in reference to a local chapter or branch, is recorded by 1799 (the verb affiliate in a related sense is from 1761). The meaning "friendship, relationship, association" is by 1852.

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hidalgo (n.)

"Spanish nobleman of secondary rank," 1590s, from Spanish hidalgo, from Old Spanish fidalgo, usually explained as a shortened from filho de algo "son" (Latin filius, see filial) "of someone" (Latin aliquis, ultimately from PIE root *al- "beyond" + PIE pronomial root *kwo-); this is perhaps an imitation of Arabic ibn-nas "son of people," a complimentary title. For alteration of f- and h- in Spanish, see hacienda.

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*dhe(i)- 

*dhē(i)-, Proto-Indo-European root meaning "to suck."

It forms all or part of: affiliate; affiliation; effeminate; effete; epithelium; fawn (n.) "young deer;" fecund; fellatio; Felicia; felicitate; felicity; Felix; female; feminine; femme; fennel; fenugreek; fetal; feticide; fetus; filial; filiation; filicide; filioque; fitz; infelicity.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dhayati "sucks," dhayah "nourishing;" Greek thēlē "mother's breast, nipple," thēlys "female, fruitful;" Latin felare "to suck," femina "woman" ("she who suckles"), felix "happy, auspicious, fruitful," fetus "offspring, pregnancy;" fecundus "fruitful, fertile, productive; rich, abundant;" Old Church Slavonic dojiti "to suckle," dojilica "nurse," deti "child;" Lithuanian dėlė "leech;" Old Prussian dadan "milk;" Gothic daddjan "to suckle;" Old Swedish dia "suckle;" Old High German tila "female breast;" Old Irish denaim "I suck," dinu "lamb."

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piety (n.)

mid-14c., piete (late 12c. as a surname), "mercy, tenderness, pity" (senses now obsolete in this word but preserved in its doublet, pity), from Old French piete "piety, faith; pity, compassion" (12c.), from Latin pietatem (nominative pietas) "dutiful conduct, sense of duty; religiousness, piety; loyalty, patriotism; faithfulness to natural ties," in Late Latin "gentleness, kindness, pity;" from pius "kind" (see pious).

From 1570s in English as "filial affection, dutiful conduct or behavior toward one's parents, relatives, country, etc." Meaning "piousness, faith in and reverence for the Supreme Being" is attested in English from c. 1600. Compare pity (n.).

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