late 14c., regreten, "to look back with distress or sorrowful longing; to grieve for on remembering," from Old French regreter "long after, bewail, lament someone's death; ask the help of" (Modern French regretter), from re-, intensive prefix (see re-), + -greter, which is possibly from Frankish or some other Germanic source (compare Old English grætan "to weep;" Old Norse grata "to weep, groan"), from Proto-Germanic *gretan "weep." "Not found in other Romance languages, and variously explained" [Century Dictionary].
From 1550s as "to grieve at (an event, action, revelation of facts, etc.)." Related: Regretted; regretting. Replaced Old English ofþyncan, from of- "off, away," here denoting opposition, + þyncan "seem, seem fit" (as in methinks).
1530s, "complaint, lament," a sense now obsolete, from the verb, or from French regret, back-formation from regreter "long after, bewail" (see regret (v.)).
From 1590s as "pain or distress in the mind due to some external circumstances" (as in to look on (something) with regret); 1640s as "pain or distress in the mind at something done or left undone." In correspondence, in declining an invitation, etc., regrets "expressions of regret, intimation of regret for inability to do something" is attested by 1851.