flourish (v.)
c.1300, "to blossom, grow," from Old French floriss-, stem of florir "blossom, flower, bloom, flourish," from Latin florere "to bloom, blossom, flower," figuratively "to flourish, be prosperous," from flos "a flower" (see flora).

Metaphoric sense of "thrive" is mid-14c. Meaning "to brandish (a weapon)" first attested late 14c. Related: Flourished; flourishing.
flourish (n.)
c.1500, "a blossom," from flourish (v.). Meaning "ostentatious waving of a weapon" is from 1550s; that of "literary or rhetorical embellishment" is from c.1600.