late 14c., peti, "small, little, minor," from a phonemic spelling of Old French petit "small" (see petit). From late 12c. in surnames. In English, not originally disparaging (as still in petty cash "small sums of money received or paid," 1834; petty officer "minor or inferior military officer," 1570s).
Meaning "of small or minor importance, not serious" is recorded from 1520s; that of "small-minded" is from 1580s. Related: Pettily; pettiness.