sucker-punch

[suhk-er-puhnch] /ˈsʌk ərˌpʌntʃ/
verb (used with object), Slang.
1.
to strike (someone) with an unexpected blow.
Related forms
sucker punch, noun
British Dictionary definitions for sucker punch

sucker punch

noun
1.
a sudden surprise punch, esp from behind
2.
a sudden unexpected defeat or setback
Idioms and Phrases with sucker punch

sucker punch

is called a sucker punch.” [ ; mid-1900s ]