Kalabasa is also used if your mouth was left wide open because of speechlessness. Kalabasa is the word used for this because when you say the word "Kalabasa" out loud, your jaw drops as you speak. (used especially after difficult quizzes, exams or even a incomprehensible joke)
A: How's the test?
B: Ayun, kalabasa. (well, I'm kalabasa.)