This is the 5501st most frequent Japanese word.
おいしかっ
"おいしかっ" is the stem of "おいしかった," meaning "was delicious."
Here, 'おいしかった' is used as a past-tense adjective to describe the cake's taste.
ケーキはおいしかったです。
The cake was delicious.
In this sentence, 'おいしかった' expresses a completed action referring to the deliciousness of food previously consumed.
パーティーで食べた料理がおいしかった。
The food eaten at the party was delicious.
Here, 'おいしかった' conveys an opinion about the food at a newly tried restaurant, emphasizing its taste in past tense.