regret (v.) Look up regret at
"to look back with distress or sorrowful longing; to grieve for on remembering," late 14c., 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, 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].

Related: Regretted; regretting. Replaced Old English ofþyncan, from of- "off, away," here denoting opposition, + þyncan "seem, seem fit" (as in methinks).
regret (n.) Look up regret at
"pain or distress in the mind at something done or left undone," 1530s, from the verb, or from Middle French regret, back-formation from regreter (see regret (v.)).