2.AdjectiveInconceivable or unimaginable; extremely improbable in a way that goes against common sense.
"Nothing is unthinkable, nothing impossible to the balanced person, provided it comes out of the needs of life and is dedicated to life's further development - Lewis Mumford"
3.AdjectiveExtremely improper; extremely at odds with norms of expected behavior.
4.NounSomething, or various things collectively, that one dare not think about, dare not do, or dare not think about doing.