Fausset's Bible Dictionary
In Roman times, their custom of reclining on Couches at meals prevailed among the Jews. Each leaned on his left arm, and so lay, as it were, in the bosom of the next below him. This position in the bosom of the master of the house was the place of honor (John 1:18; John 13:23). To lie in Abraham's bosom was thus a phrase for blessed repose in closest nearness to the father of the faithful in the feast of paradise (Matthew 8:11; Luke 16:23).
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Fausset, Andrew R. Entry for 'Abraham's Bosom'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/a/abrahams-bosom.html. 1949.