fitz n.

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."

*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."

not a native letter in Old English; in Anglo-French words it represents the "ts" sound (as in Anglo-French fiz, from Latin filius, modern Fitz); from late 13c. it began to be used for the voiced "s" sound and had fully taken that role by 1400. For letter name, see zed.

Thou whoreson Zed, thou vnnecessary Letter. ["King Lear," II.ii.69]

Series of zs to represent a buzzing sound first attested 1852; zees "spell of sleep, a nap" is slang first recorded 1963, American English student slang.