This is the 137th most frequent Dutch word.
moest
"Moest" means "had to" in English.
This uses 'moest' as the past tense of 'must' (moeten) to indicate an obligation in the past.
Ik moest mijn huiswerk maken voor de les.
I had to do my homework before the class.
Here, 'moest' introduces an unavoidable reaction in the past, implying that he found the joke genuinely funny.
Hij moest lachen toen hij de grap hoorde.
He had to laugh when he heard the joke.
In this context, 'moest' indicates a necessity dictated by circumstances (e.g., bus schedule).