This is the 1697th most frequent Dutch word.
stak
"Stak" is the past tense of "staken," which means "to strike" or "to cease work."
In this sentence, 'stak' is the past tense of 'steken', meaning 'to raise or stick out'.
Hij stak zijn hand omhoog om de buschauffeur te waarschuwen.
He raised his hand to signal the bus driver.
Here, 'stak' refers to 'oversteken', meaning 'to cross', commonly used with streets.
De man stak de straat over.
The man crossed the street.
In this case, 'stak' is part of the idiom 'niet onder stoelen of banken steken', meaning 'not to hide or suppress' opinions or feelings.