A wimp is someone who's fearful or weak. If you don't want to ride the roller coaster, you don't have to — don't let yourself be talked into it just because you're worried your friends will call you a wimp.
Wimp is an informal, derogatory way to refer to someone who's nervous or unadventurous, like calling them a "chicken" or "crybaby." And when you wimp out, you don't follow through on something because you're too scared: "I didn't mean to wimp out, but when I got up there, I changed my mind about bungee jumping." Experts aren't sure about the origin of wimp but think it may be connected to whimper, "cry softly and fearfully."