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I'm guessing that's either a reference to gambling or a caution against unproductive fun in general.

Are you sure it's not more along the lines of "don't 'play' at it, do it properly?"

Perhaps I'm reading too much into it. I guess that's the problem with this sort of vague generalised advice :)

That's how I read it, too.

If you read the second version, it is modified into "never play at any game of chance", so I think they meant gambling.