reckonyoudaoicibaDictYouDict[reckon 词源字典]
reckon: [OE] Reckon originally meant ‘give a list of, enumerate, tell’. The sense ‘count’ had developed by the 13th century, and ‘estimate, consider’ emerged in the 14th century. It comes ultimately from a prehistoric West Germanic *rekenōjan, which also produced German rechnen ‘count’ and Dutch rekenen.
[reckon etymology, reckon origin, 英语词源]
reckon (v.)youdaoicibaDictYouDict
c. 1200, recenen, from Old English gerecenian "to explain, relate, recount," from Proto-Germanic *(ga)rekenojan (cognates: Old Frisian rekenia, Middle Dutch and Dutch rekenen, Old High German rehhanon, German rechnen, Gothic rahnjan "to count, reckon"), from Proto-Germanic *rakina- "ready, straightforward," from PIE *reg- "to move in a straight line," with derivatives meaning "direct in a straight line, rule" (see regal).

Intransitive sense "make a computation" is from c. 1300. In I reckon, the sense is "hold an impression or opinion," and the expression, used parenthetically, dates from c. 1600 and formerly was in literary use (Richardson, etc.), but came to be associated with U.S. Southern dialect and was regarded as provincial or vulgar. Related: Reckoned; reckoning.