unthinkable

adjectives

incapable of being conceived or considered
unthinkable
antonym
thinkable
is similar to
totally unlikely
impossible, inconceivable, out of the question, unimaginable
see also
beyond belief or understanding
"at incredible speed"; "the book's plot is simply incredible"
incredible, unbelievable
see also
not capable of occurring or being accomplished or dealt with
"an impossible dream"; "an impossible situation"
impossible