wiseacre (n.)

1590s, partial translation of Middle Dutch wijssegger "soothsayer" (with no derogatory connotation), probably altered by association with Middle Dutch segger "sayer" from Old High German wizzago "prophet," from wizzan "to know," from Proto-Germanic *wit- "to know" (see wit (v.)). The deprecatory sense of "one who pretends to know everything" may have come through confusion with obsolete English segger "sayer," which also had a sense of "braggart" (mid-15c.).