This may be a phrase made by the author. I guess the author was influenced by 半搭+腰 when s/he composed this one.

半, 半搭, 腰 and 半腰 indicate the same meaning of half/mid/halfway/midway of some larger thing, such as a wall, or in this midplace .

It seems something larger is missing - 半搭腰 for what? You may need to add it back if you know to fully understand the meaning of this sentence. In translation, you can add it back and write like 'in the mid/midway of xxx', or just translate without this missing something and write 'fell to the midway' or something like that. But I think it may be better to find and add that something back for an translation that can be easily understood.

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Note added at 2 hrs (2008-10-23 15:44:30 GMT)
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You can see the search below to verify that this phrase is mostly a self-made one:

http://www.google.cn/search?num=100&complete=1&hl=zh-CN&neww...

Actually, as you are the translator and you can see more context, you have better chance to know the relations of the parts mentioned in this sentence:

那个室外机
那个边框
还有一个
那个半打腰

If this 室外机 is the outdoor part of an air conditioner, I would have some trouble understanding it, because the outdoor unit of my own air conditioner is encased in a single frame and supported by a housing below it. Another possibility is the unit has two (so left and right) or three (so left, right and front) "frames", with each 'frame' (a "frame" might looks more like a hollow plate or panel, I guess ) covering one side of the outdoor unit. So there is那个边框 (that frame) and还有一个 (the other).

So ”那个边框,已经松动了,已经悬了一半了” may mean the lower part of one “frame” is no longer fixed to the unit, so it looks like 悬 (hanging), but is still almost in its own position vertically, as its upper edge may still be fixed to the unit.
And “还有一个居然就已经落在那个半打腰上了” may mean another “frame” has fell down so its upper edge might leaned against the side of the outdoor unit (so what I said as the missing something is just the outdoor unit) in the mid (so 在那个半打腰上). I really cannot understand what stops it from falling further as it has already fallen that long. Maybe some part of the housing stops this “frame” at its bottom.

Just a pure guess for your reference.