dotard

[doh-terd] /ˈdoʊ tərd/
noun
1.
a person, especially an old person, exhibiting a decline in mental faculties; a weak-minded or foolish old person.
2.
doater2 .
Origin
1350-1400; Middle English; see dote, -ard
Related forms
dotardly, adverb
British Dictionary definitions for dotard

dotard

/ˈdəʊtəd/
noun
1.
a person who is weak-minded, esp through senility
Derived Forms
dotardly, adjective
Word Origin
C14: from dote + -ard
Word Origin and History for dotard
n.

"imbecile," late 14c., from dote + -ard.