Can someone help me understand this sentence?

[lucas_klumb_520717]

Hi everyone, i was looking for a sentence with つまらない to put in my anki deck and i found this one: "あの新しい映画は、つまらんかった。"
Why "つまらない" turned into "つまらん" and what is the "かった" at the end of the sentence?

"あの新しい映画は、つまらんかった。" -- "That new movie was a bummer."

Here is the site that i found the sentence: https://jisho.org/search/%E3%81%A4%E3%81%BE%E3%82%89%E3%81%AA%E3%81%8B%E3%81%A3%E3%81%9F