"old age, especially the weakness or imbecility due to old age," 1753, from senile + -ity.
Entries linking to senility
senile adj.
1660s, "suited to or characteristic of old age," from French sénile (16c.), from Latin senilis "of old age," from senex (genitive senis) "old, old man" (from PIE root *sen- "old"). The meaning "infirm from age; pertaining to or proceeding from the weakness that usually attends old age" is attested by 1882; senile dementia is attested by 1851.
-ity
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]
*sen-
Proto-Indo-European root meaning "old."
It forms all or part of: monseigneur; seignior; senate; senescent; seneschal; senicide; senile; senility; senior; seniority; senor; senora; senorita; shanachie; Shannon; signor; sir; sire; surly.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sanah "old;" Avestan hana- "old," Old Persian hanata- "old age, lapse of time;" Armenian hin "old;" Greek enos "old, of last year;" Latin senilis "of old age," senex "old, old man;" Lithuanian senas "old," senis "an old man;" Gothic sineigs "old" (used only of persons), sinistra "elder, senior;" Old Norse sina "dry standing grass from the previous year;" Old Irish sen, Old Welsh hen "old."