to act less intelligent than one is
slang; dumbed-down, adj; dumbing-down, n
Old English dumb "silent, unable to speak," from PIE *dheubh- "confusion, stupefaction, dizziness," from root *dheu- (1) "dust, mist, vapor, smoke," and related notions of "defective perception or wits."
The Old English, Old Saxon (dumb), Gothic (dumbs), and Old Norse (dumbr) forms of the word meant only "mute, speechless;" in Old High German (thumb) it meant both this and "stupid," and in Modern German this latter became the only sense. Meaning "foolish, ignorant" was occasionally in Middle English, but modern use (1823) comes from influence of German dumm. Related: dumber; dumbest.
Applied to silent contrivances, hence dumbwaiter. As a verb, in late Old English, "to become mute;" c.1600, "to make mute." To dumb (something) down is from 1933.
To make simpler and easier, esp to alter a textbook to make it more elementary • Apparently first used of movies: There has been a real ''dumbing down'' of the texts/ what some educators have called the ''dumbing down'' of textbooks/ There are jobs that will be dumbed down
[1940s+; attributed to Los Angeles Times reporter William Trombley]