I’m surprised that “slashie” in the meaning you say is the correct one, i.e. “director-slash-actor” would appear in Chicago before Los Angeles, where we have so many more such people. At least I never heard of that here in Los Angeles, although I’m not in the entertainment industry.
Since some of us haven’t the slightest idea what you are talking about, perhaps you could grace us with a link to the column in question?
The gentleman is referring to a Straight Dope Chicago column running today: http://chicago.straightdope.com/sdc20091001.php
The only time I ever heard the word vaguely in the same sense as the letter-writer uses it is in the 2001 film Zoolander. It starts with an award show in which Fabio wins a “slashie”, being a successful model/actor.
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Moved to Chicago Questions
COCC > QCFSDCC (my gods, what an acronym)
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“Slashie” isn’t a conventional term even when applied to the ubiquitous “model/actress” floating around.
Conventional, no, but the term has become fairly common since Zoolander. That was the point made in the column.