What does aegrum mean in Latin?

aegrum, a noun, means “pain”.

What is the genitive of aegrum?

The genitive singular is aegrī. aegrum is neuter, second declension.

noun · neutersecond declension

aegrum n (genitive aegrī)

neuterrarepain
Sola hic mihi nunc videor, quia ille hinc abest quem ego amo praeter omnes. Plus aegri ex abitu viri, quam ex adventu voluptatis cepi.c. 190 BCE – 185 BCE, Plautus, Amphitruo II.II.640–643
Nam gaudere novis rebus debere videtur cui veteres obsunt; sed cui nihil accidit aegri tempore in ante acto, cum pulchre degeret aevom,c. 99 BCE – 55 BCE, Lucretius, De rerum natura V.17–173

Nominalization of aeger, perhaps of older descent, see aegrōtus.