Etymology
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post office (n.)

1650s, "public department in charge of letter-carrying," from post (n.3) + office. Meaning "building where postal business is carried on, office or place where letters are received for transmission," is from 1650s. In slang or euphemistic sense of "a sexual game" it refers to an actual parlor game first attested early 1850s in which pretend "letters" were paid for by kisses.

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Definitions of post office
1
post office (n.)
a local branch where postal services are available;
Synonyms: local post office
post office (n.)
a children's game in which kisses are exchanged for pretended letters;
2
Post Office (n.)
an independent agency of the federal government responsible for mail delivery (and sometimes telecommunications) between individuals and businesses in the United States;
Synonyms: United States Post Office / US Post Office / po
From wordnet.princeton.edu