un+im+peach+able. peach sounds like "bleach" which means to whiten or to increase glow. so impeach means to decrease glow or accuse. so unimpeach means peach which is whiten or increasing glow or not accusing. having no doubts.
: impeach means to doubt or question, unimpeachable means that cannot be doubted
to PREACH is to BLAME some one or lecture them. so "unimPEACHable" is the antonym.
UNIM(not)+SPEAKABLE....when you cannot speak about a thing, its beyond question
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