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 suffixed form of PIE *dhe(i)- "to suck," with derivatives meaning "to suckle, produce, yield").
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." The meaning "skillful adroitness, admirable propriety" is attested from c. 1600.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."