weten waar Abraham de mosterd haalt

Literally, “to know where Abraham gets his mustard”. From the 18th century. The etymology is disputed, options include:

• influence from German wissen wo Barthel Most holt (“to know where Barthel gets must/wine”), which is attested somewhat earlier than the Dutch expression.
• mosterd (“mustard”) being a corruption of mutsaard (“faggot, bundle of firewood”), so the original form would literally mean “to know where Abraham gets his bundle of firewood”. The expression then refers to the Aqedah or binding of Isaac in Genesis 22.

• IPA^(key): /ʋeːtə(n) ʋaːr aːbraːɦɑm də mɔstərt fɑndaːn ɦaːlt/

weten waar Abraham de mosterd haalt

1. (idiomatic, intransitive) to be well-informed

   Synonym: van de hoed en de rand weten

   • 1789, Pieter van Woensel, Aanteekeningen, gehouden op eene reize door Turkeyen, Natoliën, de Krim en Rusland in de jaren 1784-89, vol. I, part 2, page 176:

     Zo de Godsdienst den wil des Sultans bindt, doet hij 't zeker niet dien der Keizerinne van Rusland, die zeer wel weet waar Abraham de mostaart haalt.

     (please add an English translation of this quotation)