(gingerbread doll or kind of cake for men on their fiftieth birthday)
After Abraham, the first patriarch, progenitor of the Jews.
In particular to the passage in the Gospel according to John 8:57-58: 'Thou art not yet fifty years old, and hast thou seen Abraham?' Hence 'having seen Abraham': having reached the age of fifty.
Also occurs in other expressions. “Playing Abraham”: Speaking only half the truth in a difficult situation, after Genesis 12:11-13. 'Sit in Abraham's bosom': to live a quiet, pleasant life, after Luke 16:22-23.
'He knows where Abraham gets his mustard' in the sense 'he is aware, knows all about it', does not come from the Bible. It is probably just a common name in combination with an everyday item.
(see also Sarah)
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