/ ʌnˈθɪŋkəbl; NAmE / adjective
unthinkable (for sb) (to do sth) | unthinkable (that ... ) impossible to imagine or accept
SYN inconceivable :
It was unthinkable that she could be dead.
OPP thinkable
► the un·think·able noun [ sing. ]:
Suddenly the unthinkable happened and he drew out a gun.
The time has come to think the unthinkable (= consider possibilities that used to be unacceptable) .