The plaid pattern comes from Scotland, being the original word “tartan”.
What is the difference between tartan and plaid?
The first was the pattern used to distinguish the different Scottish clans while the second was a Celtic kilt or blanket used in the Highlands, Scotland.
Since plaids are most often made from tartan cloth, the two terms are easily confusable.
Bonus curiosity: During the 18th century, the tartan was banned from the UK because it was a symbol of the Highlanders rebellion against the English monarchy.