Senile comes from Latin 'senilis' ('of an old man'), from 'senex' ('old man, old'), from Proto-Indo-European *sen- ('old'). The historical chain includes Latin senilis, Latin senex, and Proto-Indo-European *sen-. Older glosses in the chain include "senilis", "of an old man", "senex", and "old man, old", which help explain the semantic range of the modern word. Morphologically, senile is best read as borrowing, yielding an English adjective shaped by both form history and later usage. In current English it functions as an adjective, with the sense "showing the mental decline associated with old age". English derivatives include senility; related forms include senility, senate, senior, and senescence. The result is an English adjective whose etymology preserves the older forms while supporting the word's specialized modern meaning.