blaumachen
to skip work or school; to play hooky, take an unexcused day off
Origin
The origin is not entirely certain; it is usually traced to the dyers' 'blue Monday': wool dyed with woad (indigo) had to oxidise in the air and turn 'blue', so no work was done on Mondays. A second reading links 'blau' with the celebrating and drinking on the day off.
Building blocks
Words in this expression
Examples
Bei dem schönen hat die halbe blaugemacht.
With the lovely weather half the class just skipped school.