Green’s Dictionary of Slang

(US) a trick or concealed drawback.

1966		in P. Derrig Glory of the Green Berets 115: ‘But here’s the hooker, son,’ Fowler said disgustedly.
1974		Maclean’s (Toronto) Oct. 57: Canada can be trusted by nearly everyone. But — and this is the hooker — Canadian statesmen will only be effective if they speak softly.