A sip is a small amount of drink that you take into your mouth.
/ (sɪp) / verb sips, sipping or sipped. to drink (a liquid) by taking small mouthfuls; drink gingerly or delicately. noun. a small quantity of a liquid taken into the mouth and swallowed.
A "sip" is a small amount of a drink. It's less than one mouthful. When you're asking to drink someone else's drink, it's common to ask for "a sip". That's because you don't want the person to think that you're going to drink a lot of their drink!