Flowers of the Forest is an ancient Scottish lament commemorating the defeat of the Scottish army of James IV at the Battle of Flodden, the final act in the tragicomedy that was the life of James IV of Scotland, the early 16th century's latest version of that family of upper class thickos, the Stewarts.
In an attempt to suck up to Louis XII of France, Jimmy developed a Baldrickian cunning plan.
He thought he'd invade England to divert Henry VIII's troops from their campaign against the French king.
He wasn't at all deterred by the fact that he'd had his arse kicked on his two previous attempts in 1495 and 1497.
So on September 9, 1513, our man Jimmy-the-fourth invaded England with a force of more than 20,000 men. (This was 470 years before Rita May Brown pointed out the futility of doing the same thing over and over again and expecting different results, so we shouldn't be too critical.)
When the battle was over, 10,000 Scots were dead, including their vainglorious leader, who, charging on foot in the van of his men, ran downhill into the battle himself, only to find that he was no match for the English halberds.

His army was cut to pieces, and he, with the flower of Scottish nobility (not to mention the flower of Scottish Borders' male population), were slain.
Which explains the Scots pejorative phrase "Fu' yoo, Jimmy!"
"Flowers of the Forest" is a Scottish folk tune commemorating the defeat of the Scottish army of James IV at the Battle of Flodden in September 1513. It is a deeply moving lament