aegrotus.

Probably equivalent to aegrum n (“pain, distress”, noun) + -tus (“-ed”, adjective-forming suffix). However, the precise development is uncertain. The ending -ōtus, with -tus preceded by -ō-, is unique and unproductive (the usual formation would be *aegrātus, with -ātus). This suggests that Latin aegrōtus is an old formation. According to one hypothesis, some Latin adjectives in -tus (Proto-Italic *-tos) were originally built on the singular form of the Indo-European instrumental case (lost in historical Latin). Compare adjectives in -ītus and -ūtus, such as cornūtus; Nussbaum (1996 & 1998) has suggested that some words ending in these suffixes were originally deinstrumentals. See also -ētum and here. The noun aegrum (Proto-Italic *aigrom) could either be a nominalization of the neuter singular of the adjective aeger (“sick, ill, diseased”), or an older noun formation from the same root.