Someone who is either frail or cowardly can be called a weakling. When you're new to rock climbing, you may feel like a total weakling as you attempt to haul yourself up a steep, rocky incline.
Weaklings are people or animals who are weak, either in a physical or mental way. In a litter of piglets, the tiny runt is a weakling, struggling to get enough food to survive alongside its stronger litter mates. Weakling can also be a derogatory way to refer to a person who lacks physical strength or is easily intimidated: "Don't be a weakling — defend yourself against that mean gossip!"