"Picaro" is a loanword from Spanish meaning a rogue, knave, or rascal. The term implies a fun-loving lower-class person of questionable morality. In literary circles, the word "picaro" has a more specific meaning, as the name for the main character of a "picaresque" novel, and indeed it was via Spanish novels of this type that the word first entered the English language in the mid 17th century.
Etymologically, the word "picaro" is related to the French verb "piquer," the Spanish verb "picar" and the Italian verb "piccare," all meaning "to prick, to pierce, to sting," as well as the English word "picaroon" (a pirate), the English verb and noun "pique" (as in "to pique one's interest" and "a fit of pique"), and the "picador" who pricks the bull with a spear in a Spanish bullfight.