UNTENABLE

indefensible

not viable

uninhabitable

/(of theories etc.) incapable of being defended or justified
SYN - indefensible

"this was an untenable proposal and the teachers had all agreed to table it."

"the house was on the verge of being untenable, but he could put up with a lot for 150 a month."

"this was an untenable solution to their problem and he would not content himself with it."