The Spanish word 'felicísimo' meaning 'very happy' comes from combining the Spanish adjective 'feliz' (happy) with the superlative suffix '-ísimo'. The word 'feliz' itself derives from the Latin word 'felix', which meant 'happy' or 'fortunate'. The suffix '-ísimo' is used in Spanish to form absolute superlatives, indicating the highest degree of a quality - so 'felicísimo' literally means 'most happy' or 'extremely happy'.

The most common related Spanish word is the simpler form 'feliz', which means 'happy'. You might recognize this word from common phrases like 'feliz cumpleaños' (happy birthday) or 'feliz navidad' (merry Christmas). Other related words include 'felicidad' (happiness) and 'felicitar' (to congratulate).

While English doesn't have many common words directly related to 'felicísimo', we do have some words that come from the same Latin root 'felix'. These include 'felicity' (great happiness), 'felicitous' (well-chosen or suited to the circumstances), and 'felicitate' (to congratulate). The word 'felix' is also used as a given name, originally meaning 'lucky' or 'successful'.