A female in a group or in a pair, that's not attractive. The one no one wants. But since she's not attractive and doesn't get the same attention as her hotter friends she gets jealous and turns into a cock block.

A pick me is a woman that is willing to do anything for male approval. She will embarrass or throw other women under the bus to achieve this goal. The unfortunate thing about a pick me is usually the men they are trying to seek approval from are of poor quality and treat women badly, leaving little real benefit for the pick me.