The meaning of this phrase is "from beginning to end, with total knowledge", and as our colleague Felipe Lorenzo del Río explains, it seems to come from the teacher's recitation ""eme", "a", "ma!", "pe", "a", "pa!", with which the first words are taught. Although there is a version that proposes a distortion of the abbreviation "of P . a P . " ("from [initial] word to [final] word") used by medieval copyists to indicate that the transcribed book was a faithful copy. See "from end to end", "from the cross to the date".
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