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Apparently, an "extension" of the lips created when a face's cheeks are cut from the corners of the mouth to the ear, leaving a scar.

Source: blabbermouth.net, from a comment to a news article.
Tommy Flanagan, who played Cicero in "Gladiator" and Morrison in "Braveheart," has had a so-called glaswegian smile ever since being mugged at a pub at which he was employed.
by [dp] leviathan May 01, 2006