1
general::
Disclose a secret or reveal something prematurely, as in You can count on little Carol to spill the beans about the surprise. In this colloquial expression, first recorded in 1919, spill means "divulge," a usage dating from the 1500s.
American Heritage Idioms
2
general::
Phrase(s): spill the beans [and] spill the works
Fig. to give away a secret or a surprise. • There is a surprise party for Heidi on Wednesday. Please don’t spill the beans. • Paul spilled the works about Heidi’s party.
McGrawhill's American Idioms And Phrasal Verbs