Senile means weakened in mind or body by very great age. A word medicine has largely abandoned.
It was once a neutral clinical term, and senile dementia was a diagnosis, but the word became an insult and doctors now say dementia, or name the specific condition. Using it about a person is unkind and imprecise, since confusion in old age has causes that can often be treated. It survives neutrally in a few technical phrases. The noun senility carries the same problem, and old age is the plain alternative.
1adjectivesenileSuffering mental or physical decline from old age.
“The old term senile dementia has fallen out of use.”
Senile means nothing worse than old. Latin senex was an old man and senīlis meant belonging to old age, so the judgement English hears in the word is entirely ours.
Proto-Indo-European
*sénos — old
Latin
senīlis — of or pertaining to old age
Old French
senile
Middle French
senile
French
sénile
English, today
senile
Built from
1660s; borrowed from French sénile, from Middle French senile, from Old French senile, from Latin senīlis (“of or pertaining to old age”), from senex (“old man”), ultimately from Proto-Indo-European sénos (“old”).
Etymology from Wiktionary, CC BY-SA