
Why is it that people who cannot speak are called 'dumb'? Hence the term deaf and dumb. I've seen that it can be considered offensive to some people.
First, yes, definitely offensive. Right? Not the student's question, but the label 'deaf and dumb' is clearly awful.
The historical question is interesting, though. I had a theory that maybe this might have been connected to antiquated beliefs about Deaf people, and prejudices against them, that may have lead the word 'dumb' in its sense as 'ignorant, stupid' to be applied to those who didn't speak.
And I'm old enough to remember the phrase 'deaf-mute' replacing 'deaf and dumb' as the polite way of referring to a Deaf person. I think that 'deaf-mute' is also an inappropriate way to label a Deaf person...but that it replaced 'deaf and dumb' suggested to me that the 'dumb' part must have come from the pejorative meaning of the word.
Turns out I was wrong.
The word 'dumb' has its roots in Old English, with the earliest attestations around 1000CE. It came into OE probably from Old Norse, or perhaps from Gothic - and the sense was (only) 'without speech'. Cognates in the other Germanic languages may have also had a connotation of 'ignorant, stupid', but this sense was not evident in Old English, Old Norse or Gothic.
The earliest citation for 'dumb' in the OED (2016) is this, in which it refers to a man who was without speech:
c1000 West Saxon Gospels: Matt. (Corpus Cambr.) ix. 32 Hig brohton hym dumbne man
This first usage of 'dumb' in English did occur with 'deaf' - so 'deaf and dumb' is a very old label indeed.
From this usage, the word was extended as early as the 1200s to apply to non-human animals, inanimates, and forces of nature - still meaning 'without speech'. It was also extended to refer to those who were shy, taciturn or reticent (who choose not to speak), and those who lack influence in their lives or in the public sphere (today we'd say they 'lack a voice').
It isn't until the 1750s that we see usages of 'dumb' in English in the sense 'stupid, ignorant'; and then not really robustly until the 1800s.
And you can see the trajectory of the competing phrases 'deaf-mute' (with and without the hyphen) vs. 'deaf and dumb' in Google's corpus of English books (Google ngram viewer, 2016) below. Both phrases have declined precipitously in the last 50 years, which I would argue is a good thing.
And I would also assume that for most contemporary speakers, the 'dumb' in 'deaf and dumb' is impossible to detach from the connotation of mental deficiency. Of course it's an inappropriate thing to label someone...
but isn't it interesting how things change?
Google ngram viewer. 2016. "deaf-mute, deaf mute, deaf and dumb". Google Ngram Viewer Team. https://books.google.com/ngrams (accessed August 30, 2016).
OED Online. 2016. "dumb, adj. and n.". Oxford University Press. http://www.oed.com.ezproxy1.library.arizona.edu/view/Entry/58378?rskey=fEFcYh&result=1 (accessed August 30, 2016).