D defs.my
Entry 1 sense Webster, 1913

Reckoner

/(rĕk"'n*ẽr)/ · Reck·on·er · IPA /ˈɹɛkənə(ɹ)/
01 n. One who reckons or computes; also, a book of calculations, tables, etc., to assist in reckoning.
  1. 1.
    One who reckons or computes; also, a book of calculations, tables, etc., to assist in reckoning.
    Reckoners without their host must reckon twice.” — Camden.