The phrase "pick at" is a common term used to describe someone who's constantly nagging or criticizing. It can also be used to describe someone who's constantly fidgeting or playing with something, such as a piece of clothing or a fingernail. Synonyms for "pick at" include "nag", "nitpick", "criticize", "fuss over", "pester", and "bother". These words can be used interchangeably in order to add more variety to your writing or speech. Regardless of the term used, the essence of "pick at" remains the same - a behavior that can be very annoying and disruptive.