Sean (written "Seán" or "Séan" in Irish) is a Hibernization of the English name "John"; that is, it's a transliteration of "John" into a form which can be pronounced in Irish and written with the Irish alphabet (which nowadays is simply a version of the Roman alphabet).
There were references to the origin of the above idiom, my favorite (supported here as well) being from the movie, The Untouchables, wherein Sean Connery utters with contempt, "Isn't that just like a [racist for Italian]... brings a knife to a gun fight."
I've noticed many Scottish and Irish Gaelic words to be spelled with an s, followed by a vowel, and pronounced like sh. Think about the way Sean Connery speaks (not to mention how the Se in his name is pronounced).
Sean, above, wrote, "free is just a placeholder for $0." I disagree, and this is the point. The term 'for' must be used with a commodity. The use of a commodity, such as 'five dollars', can be correctly phrased, "for five dollars". It's an amount. But the term 'free' denotes the ABSENCE of a commodity. 'Free' denotes amountlessness.
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