die Spendierhosen anhaben
to be in a generous mood; to be willing to treat others
Origin
The etymology is uncertain; the image of 'generosity trousers' (with bulging pockets) appears as early as around 1700 in Abraham a Sancta Clara and was probably spread by student slang. It may rest on a reinterpretation from Low German.
Building blocks
Words in this expression
Examples
Heute zahle ich die – ich habe die Spendierhosen an!
Today I'm buying the round — I'm feeling generous!