issimum

Issimum is the neuter singular form of a Latin superlative adjective ending. It is not a standalone word in Latin but the tail end of the suffix -issimus used to form the highest degree of adjectives. In classical Latin, adjectives take the endings -issimus, -issima, -issimum for masculine, feminine, and neuter respectively, to express “the most” or “very.”

Examples of the full superlative forms include fortissimus (the bravest), altissimus (the tallest), and amabilissimum (the most agreeable). The neuter singular form, fortissimum or altissimum, is used when the noun it modifies is neuter or when the phrase is used predicatively about a neuter thing. It can also appear as a substantive when referring to a neuter noun conceptually described as the most X.

In ordinary Latin usage, issimum appears primarily as part of a word rather than as an independent lexeme with its own meaning. Dictionaries and grammar references describe issimum as the neuter singular ending of the superlative formation rather than as a separate vocabulary item. Its interpretation depends entirely on the stem to which it is attached and the noun it accompanies.

Summary: issimum is a grammatical form rather than a distinct word, representing the neuter singular of the Latin superlative suffix -issimus and signaling the most or very in relation to a neuter noun.