The only free and comprehensive online etymological dictionary of the Spanish language
feliz (Adjective) "happy"
13th cent. From Latin felicem, accusative of felix "happy," but earlier "fruitful." From Proto-Italic *fēlwī-, which de Vaan (2014) evincingly interprets as "with young." From Proto-Indo-European *dheh1-lu̯-i- 'id.,' a hypercorrection of earlier *dheh1-lu̯- 'id.' From the noun *dheh1-l- "young animal." From the root *dheh1- "to suckle."
As there were myriad Catholic saints named Felix, myriad names and surnames appeared throughout Spain in Christian reverence: Feliz, Félez, Felices, Felies, Felis, San Félix, Félix, San Félix, Santelices, Sanfelis, and Sanfiz.
Indo-European
Romance
Asturian feliz, Portuguese feliz, Galician feliz, Catalan feliç, Italian felice, Romanian ferice
Celtic
Middle Irish deil "sow"
Germanic
Old High German tila "nipple," Old English delu 'id.'
Balto-Slavic
Lithuanian dėlė̃ "leech," Latvian dēle 'id.'
Hellenic
Ancient Greek θῆλυς (thêlus) "female"
Armenian
Armenian dayl "beestings"
Indo-Iranian
Sanskrit dhārú- "suckling"