USA, 1966—a (13th-birthday) party held for a dog—a blend of ‘bark’ (the sharp explosive cry of a dog), and of ‘bar mitzvah’ (the coming-of-age ceremony for a 13-year-old Jewish boy), or ‘bat mitzvah’ (the equivalent ceremony for a Jewish girl)
USA, 1843—With reference to a dog used for hunting game, this phrase is used to express the opinion that a particular plan or approach will not succeed.
colloquial, Australia, 1910—‘the night’s (only) a pup’, or ‘the day’s (only) a pup’, means the night, or the day, is young—refers to ‘pup’ in the sense of a young dog
1950—used of a substance causing death or illness, and by extension of something powerful or disastrous—refers to red kelpie sheep dogs, who can ingest anything
far-fetched excuse for failing to hand in school homework—1st recorded UK 1929 but had already long been in usage at that time—dog eating a sermon UK 1894
UK, 1980s—the very best—perhaps from ‘it sticks out like a dog’s ballocks’, denoting something obvious, hence someone or something that sticks out from the rest
UK, 1882—to remain motionless and quiet; to keep a low profile—probably from ‘dog’ and suffix ‘-o’, with allusion to the characteristically light sleep of a dog