-íssimo

Borrowed from Latin -issimus. The borrowing occurred during the Renaissance by influence of Italian^[1].

-íssimo m (feminine -íssima, plural -íssimos, feminine plural -íssimas)

1. forms the absolute superlative

   ‎caro (“expensive”) + ‎-íssimo → ‎caríssimo (“extremely expensive”)

   ‎feio (“ugly”) + ‎-íssimo → ‎feiíssimo (“extremely ugly”)

• Though both are called superlatives, Portuguese words suffixed with -íssimo are not equivalent to English words suffixed with -est or preceded by most. Instead, they are equivalent to using the adverbs very or extremely to qualify the adjective.
• Adjectives ending in -ável or -ível are suffixed with -abilíssimo or -ibilíssimo instead, replacing the suffix:

  ‎aceitável (“acceptable”) + ‎-íssimo → ‎aceitabilíssimo

• Adjectives ending in m change it to n:

  ‎comum (“common”) + ‎-íssimo → ‎comuníssimo

• Adjectives ending in -ão change it to -an:

  ‎são (“sane”) + ‎-íssimo → ‎saníssimo

• Adjectives ending in z change it to c:

  ‎feliz (“happy”) + ‎-íssimo → ‎felicíssimo.

• Adjectives ending in the semivowel <i> followed by a vowel lose the semivowel, in nonstandard usage:

  ‎feio (“ugly”) + ‎-íssimo → ‎feíssimo (nonstandard)

  ‎feio (“ugly”) + ‎-íssimo → ‎feiíssimo (standard)

• Some words may loan the superlative from Latin in addition to forming one with this suffix:

  paupérrimo (loaned from pauperrimus) and pobríssimo, superlatives of pobre

• Colloquially, the syllable <ssi> may be repeated a number of times for emphasis:

  ‎grande (“big”) + ‎-issíssimo → ‎grandissíssimo (“very very big”)