Quote Originally Posted by gilmore
What is the source for this?
The actual tartan proscription act read as follows -

"That from and after the First Day of August 1747, no man or boy within that part of Great Britain called Scotland, other than such as shall be employed as Officers and Soldiers of His Majesty's Forces, shall on any pretext whatsoever, wear or put on the clothes, commonly called Highland clothes (that is to say) the Plaid, Philabeg, or little kilt, Trowes, Shoulder-Belts, or any part whatever of what peculiarly belongs to the Highland Garb; and that no tartan or party-coloured plaid or stuff shall be used for Great coats or upper coats, and if any such person shall presume after the first said day of August, to wear or put on the aforesaid garments or any part of them, every person so offending.... shall be liable to be transported to any of His Majesty's plantations beyond the seas, there to remain for the space of seven years."

What Foxgun Tom has quoted is what is commonly known as the 'Highlander's oath'.