Plutarch's explanations were interesting and humorous about Haw, get-up, and gee. Interestingly, the AOD gives only the meaning of saying "gee" to command a horse to go faster. So their gee and your get-up [or giddyup] are the same. The AOD would have gotten more at the meaning of agreement or meshing or getting along together from the OED's citations of "gee"--even though we're talking UK/US.
Of course, in context, we're not talking about "Gee" or "Gee whiz" here but agreement, which seems to have brought about quite a bit of etymological hem-hawing and not a few wags:).--Johnnie G

Johnnie Godwin

Johnnie Godwin