As nouns the difference between dunderhead and underhead
is that dunderhead is a stupid person; a dunce while underhead is a blockhead, or stupid person; a dunderhead.
As an adjective underhead is
beneath the head of a mechanical component.
underhead
English
Noun
(
en noun)
(obsolete) A blockhead, or stupid person; a dunderhead.
-
Adjective
(
-)
(engineering) Beneath the head of a mechanical component.
(
Webster 1913)