senectitude (si-NEK-ti-tood, -tyood) noun Old age. [From Latin senectus (old age), from senex (old). Ultimately from the Indo-European root sen- (old) that's also the ancestor of senior, sir, sire, senate, senile, Spanish señor, and surly (which is an alteration of sirly, as in sir-ly).] -Anu Garg... Sign in to see full entry.