The usage of “-isimo” and its variants in Romance languages originates from Classical Latin. This shows how linguistic elements can last through centuries of language evolution, even as the languages themselves diverge and develop unique characteristics.
The Latin suffix to indicate the superlative degree of adjectives “-issimus” was inherited by Spanish, Italian, Portuguese, and Catalan (no French in this case).
These are 10 examples of superlative forms for each language: Spanish, Italian, Portuguese, and Catalan. (masculine, singular).
English
Spanish
Italian
Portuguese
Catalan
Very good
buenísimo
buonissimo
boníssimo
boníssim
Very big
grandísimo
grandissimo
grandíssimo
grandíssim
Very small
pequeñísimo
piccolissimo
pequeníssimo
petitíssim
Very easy
facilísimo
facilissimo
facílimo
facilíssim
Very beautiful
bellísimo
bellissimo
belíssimo
bellíssim
Very fast
rapidísimo
rapidissimo
rapidíssimo
rapidíssim
Very strong
fortísimo
fortissimo
fortíssimo
fortíssim
Very difficult
dificilísimo
difficilissimo
dificílimo
dificilíssim
Very expensive
carísimo
carissimo
caríssimo
caríssim
Very rich
riquísimo
ricchissimo
riquíssimo
riquíssim
Each of these examples shows how the superlative form is constructed in these four Romance languages using the “-ísimo/-issimo/-íssimo/-íssim” suffix. The words might have slight meaning differences between languages, in Italian, you would call a dear friend carissimo.
Since Spanish adjectives ending in “o” can take 4 forms:
Masculine Singular:
El libro es buenísimo. (The book is very good.)
Feminine Singular:
La película es buenísima. (The movie is very good.)
Masculine Plural:
Los libros son buenísimos. (The books are very good.)
Feminine Plural:
Las películas son buenísimas. (The movies are very good.)
Sure, you also have the option to say muy bueno, muy fácil, muy caro, muy rico instead. I would say buenísimo is used more in spoken form, for example, in TV commercials.
Evolution of the “-ísimo” Latin suffix
Latin Origin:
In old Latin, they used the endings “-issimus” for masculine, “-issima” for femenine, and “-issimum” for things.
For example: “altus” (high) becomes “altissimus” (highest).