/ prɪˈpɒstərəs; NAmE -ˈpɑːs-/ adjective ( formal )
1.
completely unreasonable, especially in a way that is shocking or annoying
SYN outrageous :
These claims are absolutely preposterous!
2.
unusual in a silly or shocking way
SYN outrageous :
The band were famous for their preposterous clothes and haircuts.
► pre·pos·ter·ous·ly adverb :
a preposterously expensive bottle of wine
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WORD ORIGIN
mid 16th cent.: from Latin praeposterus reversed, absurd (from prae before + posterus coming after) + -ous .