late 13c., demaunde, "a question," from Old French demande, from demander "to request; to demand" (see demand (v.)). Meaning "a request, a claim, an asking for by virtue of a right or supposed right to the thing sought," also "that which is demanded or required, exaction as a tribute or concession," without reference to right, is from c. 1300.
In the political economy sense of "desire to purchase and possess, coupled with the means to do so" (correlating to supply) it is attested from 1776 in Adam Smith. Meaning "state of being sought after" (especially by consumers) is from 1711. In demand "much sought after" is attested by 1825; on demand "on being requested" is from 1690s.