1580s, from Late Latin ventriloquus, from Latin venter (genitive ventris) "belly" (see ventral) + loqui "speak" (see locution). Related: Ventriloquial; ventriloquize.
Patterned on Greek engastrimythos, literally "speaking in the belly," which was not originally an entertainer's trick but rather a rumbling sort of internal speech, regarded as a sign of spiritual inspiration or (more usually) demonic possession. Reference to the modern activity so called seems to have begun early 18c., and by 1797 it was being noted that this was a curiously inappropriate word to describe throwing the voice.