Green’s Dictionary of Slang

1. a euph. for canine excrement.

1972		B. Rodgers Queens’ Vernacular.
1995		P. Vautin Turn It Up! 80: One day I put a fake doggie doo under a girl’s chair, waited ten minutes and shouted, ‘Oh, look what Tracey did’.
2002		‘The Last Word’ 22 Jan. on Slam! Sports 🌐 [He] got surly on Hockey Night In Canada on Saturday night. Of course, when does he ever not look like he just stepped in doggy-do.

2. thus by ext. anything, e.g. food, that is disgusting.

1972

B. Rodgers Queens’ Vernacular 65: doggy-do [...] 2. bad-tasting food ‘I can’t eat this doggy-do. Call the waiter and tell him to sweep it away!’.