Crossword Genius

Like Molly Brown with a lisp? Highly unlikely (11)

Ross

I believe the answer is:

unthinkable

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'unlikely' is the definition.
(thesaurus)

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(Other definitions for unthinkable that I've seen before include "it's out of the question" , "Too unlikely to consider" , "Inconceivable" , "Preposterous" .)

I've seen this clue in The Daily Mail.
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