A pincer is the claw-like appendage on an insect or crustacean that allows it to grab things, particularly food. Your crab will never be a cuddly pet, because of those pincers you have to watch out for. Ouch!
If someone catches lobsters for a living, she'll have to be very careful of all of those pincers — in fact, when you buy live lobsters, their pincers are usually restrained with rubber bands. The danger with pincers is that they'll pinch, and these words share a root, the Old French pincier, "to pinch." There's also a tool called pincers that's modeled on animal pincers and used to grip, pull, or cut something.