"Italian lord or gentleman," 1570s, from Italian signore, from Latin seniorem, accusative of senior "older" (from PIE root *sen- "old"). Sometimes English uses the full Italian form. Feminine form signora is attested in English from 1630s; diminutive signorina is recorded from 1820.
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."
late 13c., "the elder," from Latin senior "older," comparative of senex (genitive senis) "old" (from PIE root *sen- "old"). Its original use in English was as an addition to a personal name indicating "the father" when father and son have the same name (e.g. Walterus Baddyng, seniore in late 13c. Leet rolls of the City of Norwich). The meaning "higher in rank, longer in service" is recorded by 1510s.
The Latin word yielded titles of respect in many languages, such as French sire, Spanish señor, Portuguese senhor, Italian signor. Also compare Herr. Senior citizen "elderly person" (typically one past retirement age) is by 1938, American English.