felicity: [14] Felicity and its relatives felicitous [18] and felicitate [17] all come ultimately from Latin fēlīx. This originally meant ‘fruitful’ (it is probably related to English fecund and foetus) but by classical times had progressed semantically via ‘favourable, fortunate’ to ‘happy’. => fecund, foetus
felicity (n.)
late 14c., "happiness; that which is a source of happiness," from Old French felicite "happiness" (14c.), from Latin felicitatem (nominative felicitas) "happiness, fertility," from felix (genitive felicis) "happy, fortunate, fruitful, fertile," from Latin root *fe-, equivalent of PIE *dhe(i)- "to suck, suckle, produce, yield" (see fecund). Meaning "skillful adroitness, admirable propriety" is from c. 1600. A relic of Rome's origins as an agricultural community: that which brings happiness is that which produces crops. Compare pauper (see poor (adj.)) "poor, not wealthy," literally "producing little."