Etymology
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reckoner (n.)

c. 1200, rekenere, "one who keeps accounts or computes," agent noun from reckon (v.). Later especially "an aid in reckoning, something that assists a person to reckon accounts;" especially "book of tables used in calculation," often known as a ready reckoner (1757).

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Definitions of reckoner

reckoner (n.)
an expert at calculation (or at operating calculating machines);
Synonyms: calculator / figurer / estimator / computer
reckoner (n.)
a handbook of tables used to facilitate computation;
Synonyms: ready reckoner
From wordnet.princeton.edu