dotard

Etymology

From Middle English dotard; equivalent to dote + -ard.

noun

1. An old person with impaired intellect; one in their dotage.
   Lawns, houses, chattels, groves, and fields, / All that the fertile valley shields; / Wages of folly--baits of crime, / Of life's uneasy game the stake, / Playthings that keep the eyes awake / Of drowsy, dotard Time; 1835, William Wordsworth, “The Pass of Kirkstone”, in A Guide through the District of the Lakes
   1867, W. S. Gilbert, "The Precocious Baby," The 'Bab' Ballads, Complete Edition, Philadelphia: David McKay, no date, p. 73, https://archive.org/details/babballads00gilb He early determined to marry and wive, / For better or worse / With his elderly nurse, / Which the poor little boy didn't live to contrive: / His health didn't thrive— / No longer alive, / He died an enfeebled old dotard at five!
   Whatever [Donald J.] Trump might have expected, he will face results beyond his expectation. I will surely and definitely tame the mentally deranged U. S. dotard with fire. 2017-09-22, Kim Jong-un, quotee, “Statement of Chairman of State Affairs Commission of DPRK”, in KCNA Watch
2. One who dotes on another, showing excessive fondness.