adverb, noun
most ... ever; most ... up to now; more ... than ever before
Slang abbreviation of 過去一番 (かこいちばん). Used adverbially or as a noun to express that something is the most extreme in one's experience up to now. Often followed by an adjective or adjectival noun.
このラーメン、過去一うまい！
This ramen is the best I've ever had!
今日のテスト、過去一難しかった。
Today's test was the hardest ever.
The full, non-abbreviated form. 過去一 is a slangy shortening used in casual speech and online writing.
Abbreviation of 過去一番 (かこいちばん), literally 'past number one'.