to assume an answer to an unstated question or premise
c.1200, perhaps from Old English bedecian "to beg," from Proto-Germanic *beth-; or possibly from Anglo-French begger, from Old French begart (see beggar). The Old English word for "beg" was wædlian, from wædl "poverty." Of trained dogs, 1816.
As a courteous mode of asking (beg pardon, etc.), first attested c.1600. To beg the question translates Latin petitio principii, and means "to assume something that hasn't been proven as a basis of one's argument," thus "asking" one's opponent to give something unearned, though more of the nature of taking it for granted without warrant.
To assume what has still to be proved: “To say that we should help the region's democratic movement begs the question of whether it really is democratic.”