-íssimo

See also: -issimo

Portuguese

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

-íssimo (adjective-forming suffix, feminine -íssima, masculine plural -íssimos, feminine plural -íssimas)

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.
The following ending(s) change(s) in words appended with this suffix:
- ⟨-ão⟩ (as a non-verb suffix, including plurals) → ⟨-on-⟩
- ⟨-ã(o)⟩ (including plurals) → ⟨-(i)an-⟩, ⟨-am-⟩, or ⟨-(i)on-⟩, depending on the base word's etymology
- ⟨-m⟩ (including plurals) → ⟨-n-⟩
- ⟨-vel⟩ (unstressed and adjectival, including plurals) → ⟨-bil-⟩
- ⟨-z⟩ (adjectival, including plurals) → ⟨-c-⟩
Adjectives ending in the semivowel ⟨i⟩ followed by a vowel lose the semivowel in nonstandard usage:
‎feio (“ugly”) + ‎-íssimo → ‎feíssimo (proscribed)

‎feio (“ugly”) + ‎-íssimo → ‎feiíssimo (prescriptive)
Some words may loan their superlative from Latin in addition to forming one with this suffix:
paupérrimo (from Latin pauperrimus) coexists with pobríssimo as superlatives of pobre
Colloquially, the syllable ⟨ssi⟩ may be repeated a number of times for emphasis:
‎grande (“big”) + ‎-issíssimo → ‎grandissíssimo (“very very big”)

^ 2009, Evanildo Bechara, Moderna Gramática Portuguesa, 37^th edition, Editora Nova Fronteira, Editora Lucerna, page 154.