What is the meaning of -issimo?

Added to an adjective, creates an adjective. This suffix is an intensifier which adds a qualifier of "extremely" or "remarkably."

added to adjectives to form absolute superlatives

added to adverbs to form absolute superlatives

added to some nouns to form extremes

added to the stem of verbs ending in -ire to form the first-person plural imperfect subjunctive tense

superlative degree of -ō

Source: wiktionary.org