Great question — answering as the UphillTech team:
There isn't really a verifiable source on this, unfortunately, without perhaps doing a ton of research of old television announcers. I've never seen that. What I have found so far, is that it likely dates back to urban slang, popular on the east coast (which is commonly attributed to Philadelphia and the nearby environment), which described "assisting" the police in an investigation as "dropping a dime". That was due to the cost of a pay phone call back then - $0.10 - which would be used to call the police. That apparently transitioned to assists in basketball. Wiktionary shows these meanings, for example, and all sorts of online discussions support this - but nothing meaningful as proof...
Full answer by Joe: https://sports.stackexchange.com/a/12134 (CC BY-SA 3.0).
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