dumb [OE] In Old English dumb signified ‘unable to speak’, and could apply to both humans and animals (dumb beasts). The sense ‘stupid, unintelligent’ dates from the Middle Ages. The original meaning does, however, lie behind the dumb- bell [E18th], which originally referred to an apparatus similar to that used to ring a church bell but without the bell making it therefore noiseless or ‘dumb’. It is also behind dummy [L16th]. The original sense was ‘a person who cannot speak’, then [M18th] ‘an imaginary fourth player in whist’. This gave rise to ‘a substitute for the real thing’ (e.g. a rubber teat, a blank round of ammunition), and ‘a model of a human being’ (mid 19th century).

A person considered stupid began to be called a dumbo in the USA in the 1950s, probably inspired by the 1941 Disney cartoon film Dumbo, which featured a flying elephant. The elephant’s name was probably based on *jumbo. Worry about things being dumbed down, or having their intellectual content reduced so as to be accessible to a larger number of people, seems very recent, but the phrase goes back to 1920s. See also dizzy, limb.