This suffix -issime comes from the Latin -issimus and means “the most”.
It is possible to make it with all adjectives but some are more common/
Simplissime (very easy)
Grandissime (very big)
Gravissime (very serious)
Rarissime (very rare)
Richissime (very rich)
And those that have an Italian origin are like -issimo:
We also have the word: bravissimo like in English!