Hi Meantolearn,
I find this very funny because my sister often tells me to get my
butt in gear! Sometimes she tells me to get my fat butt in gear! I can
almost hear her sweet voice now... "Get your fat butt in gear or we'll
be late! Come on, we haven't got all day!" All of the meanings you
mention are related - "move", "get ready", "be in a hurry". If I'm
in a hurry, I have to finish getting ready quickly, don't I? If I'm in
a hurry, I have to move quickly, right? So I have to be like a race car
driver and "get myself in gear".
In the original ad you mentioned, Dell wants you to hurry up
and move, which to them means "move towards the telephone and
order some of our wonderful 'gear'!". "Move" doesn't have to
be physical movement. It could be a kind of mental "action".
For example, if I just took a difficult test, I might say, "I had
trouble with the first few questions, but then I got my butt in gear
and I was all right". This means that I struggled with the first few
questions, I was "moving" slowly. Then my super "motorized" brain
got in gear, and I was all right!
One thing about idioms such as this is that you should be careful in
using them. If you use them at the wrong time, you could easily be
understood. If, for example, you were to say "Get your fat ass in
gear!" to the wrong person, you could find yourself in the middle of a
big fight! It's great to understand these kinds of expressions but you
don't really need to use them. I don't think I've ever said "get in
gear", in any form, in my life. Perhaps someday I'll say it,
once, maybe even twice. Or maybe not. It's not really
necessary.