What does ūndēcentum mean in Latin?

ūndēcentum, a numeral, means “ninety-nine; 99”.

How do you pronounce ūndēcentum?

In classical Latin, [uːn.deːˈkɛn.tũː]; in ecclesiastical (church) Latin, [un.deˈt͡ʃɛn.tum].

numeral

ūndēcentum (indeclinable)

Classical[uːn.deːˈkɛn.tũː]Ecclesiastical[un.deˈt͡ʃɛn.tum]

indeclinableninety-nine; 99
nec congruēbant ad hōrās eius līneae, pāruērunt tamen eī annīs ūndēcentum, ...Although the threads did not come together for those hours, they were evident those ninety-nine years, ...77-78 — Pliny the Elder, Naturalis Historia, Book VII, section 214

undēcentum, num. adj. unus-decentum,

I. ninety-nine: anni, Plin. 7, 60, 60, § 214.

Literally "one from one hundred"; ūnus (“one”) + dē (“from”) + centum (“hundred”)