felicísimo
happiest
feliz
happy
+
-ísimo
superlative suffix
Felicísimo means happiest and can be broken down into feliz0 (happy0) + -ísimo1 (superlative suffix1), literally meaning 'most happy'.
adjective
- happiestExtremely happy or joyfulEstoy felicísimo con los resultados del examen. // I am overjoyed with the exam results.