This scene always bothers me, because I know the term had been around for a while at this point. But would it have been pervasive enough to reach the Scottish Highlands?
Without having any knowledge of the show or its use the scene, the earliest recorded Scottish use is a 1503 poem by William Dunbar, followed by Scots satirist Sir David Linsay who wrote of the hypocritical clergy in 1535 that they "...may fuck thair fill, and be vnmarryit." ("...may fuck their fill, and be unmarried.")
The origin of the 'English' word may well be Scottish, via the Old Norse for "drive" (and the Highlands fell well within that sphere of influence), as Scotland represents the bulk of uses across the 16th Century, but the trend is towards use by the educated classes, best exemplified by Charles II's favourite syphilitic rake, John Wilmot, 2nd Earl of Rochester, in 1674:
Peace is his aim, his gentleness is such,
And love he loves, for he loves fucking much.
and 1671-6:
"Is't so?" quoth the Duchess. "Aye, by God!" quoth the whore.
"Then give me the key that unlocks the back door,
For I'd rather be fucked by porters and carmen
Than thus be abused by Churchill and Jermyn."
Maybe our hero, being a romanticised Jacobite highlander spoke Gaelic as his first language and not Scots, but he would mostly likely be a fluent speaker of both had he a mind to travel beyond his village.
My primary source is An Encyclopedia of Swearing: The Social History of Oaths, Profanity, Foul Language, and Ethnic Slurs in the English-speaking World by Geoffrey Hughes
The word was used by Scottish poet William Dunbar in a 1510 poem 'A Brash of Wowing' in the form fukkit. Dunbar would have expected his audience to be familiar with it at least in context. The Scottish guy should have known the word.