Thank you for using the Jiskha Homework Help Forum. When you look in a dictionary for the meaning, you will just find the infinitive:
picar = to prick, pierce, puncture; sting; bite; burn; peck; nibble; pit, pock; mince, chop up, cut up; stick, poke, spur; goad; perforate; crack (as ice); harass, pursue; tame; pique, annoy. ALL those meanings, depending upon the context, or exactly how the word is used.
piques = Present Subjunctive, tú form
picas = Present Indicative, tú form
pica = either a tú affirmative command OR 3rd person singular, Present Indicative
piqué = yo form of the Preterit
¿Pico la cebolla? is asling "Do I cut the onion up? If you wish to say no, plus a command to someone to NOT do that = no, no la piques (tú command) or no la pique (formal command).