weten waar Abraham de mosterd haalt

Etymology
Literally, “to know where Abraham gets his mustard”. From the 18th century. The etymology is disputed, options include:
 * influence from German, which is attested somewhat earlier than the Dutch expression.
 * being a corruption of, so the original form would literally mean “to know where Abraham gets his bundle of firewood”. The expression then refers to the or binding of Isaac in Genesis 22.

Verb

 * 1)  to be well-informed