Green’s Dictionary of Slang

pear-shaped adj.

[the image of a solid rectangle ‘slipping down’ into a pear shape, hence ‘the bottom drops out’; adj. is backform. f. phr. below]

out of order, going badly or wrong.

[UK]N. Barlay Crumple Zone 190: If you get ’em angry iss all-pear-shaped.

In phrases

go pear-shaped (v.) (also pear-shape, turn pear-shaped)

of plans or schemes, to fail, to collapse.

[US] Hip-Hop Connection Dec. 16: Just like with all families, it can all go pear-shaped later on.
[UK]Guardian Weekend 30 Oct. 100: Last year it went a bit pear-shaped.
[UK]B. Hare Urban Grimshaw 210: This one could turn pear-shaped. If the victim dies, you’re looking at conspiracy to murder.
[Aus]P. Temple Broken Shore (2007) [ebook] Sacked three weeks ago. [...] Someone leaked information and the whole thing went-pear-shaped. They blamed me.
[SA]Mail & Guardian 13 Apr. 🌐 I can sense this interview is going pear-shaped.
[Aus]G. Disher Heat [ebook] ‘At least we got out before anything went pear-shaped’.
[UK]M. Herron Secret Hours 155: ‘If this goes pear-shaped for any reason, there’ll be no doubt in my mind who’s responsible’.
[UK]J. Meades Empty Wigs (t/s) 747: [H]is reputation pear-shaped when he penned a disgusting sounding book The Riddle of the Sphincter.