Example sentences:

Ramayana: Ravana dismissed as preposterous the notion that stones would float to form a bridge — till he saw hundreds of monkeys charging into his island. 

Mahabharata: Bhishma declared that Duryodhana’s plan to arrest Krishna was preposterous; Krishna was the undefeatable supreme descended to the earth.  

Bhagavad-gita: Some people consider belief in God preposterous, others consider disbelief in God preposterous.