I don't think this is an English idiom.
It's used mainly in German forums, and I've found an explanation from someone:
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Perhaps it was something like an impossible translation of
the German "
schwer auf Draht sein" into "to be heavy on the wire".
It should mean, to be top concentrated, highly ambitioned and best
prepared.
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Perhaps: intense, focussed.
OK, I've found another site:
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schwer auf Draht sein to be heavy on wire meaning:
to react very fast
http://www.wolf-zone.de/projects/german/language.html
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