Little pickle boy can mean a number of different things, most commonly it is used as a nonsensical insult to demean a person in the least hurtful way, a silly phrase used to cheer up someone or just a joke to get a group to start laughing randomly.
The ‘boy’ in little pickle boy is also optional.
Person 1: *does anything*
Person 2: “You know what you’re a little pickle boy, la la la la little pickle!”
Person 1: “I don’t no man, it seems like every day keeps getting worse than the last”
Person 2: “Little pickle, la la la!”
Both: *starts laughing*
Person: “Hey little pickle boy”
Group: *starts laughing*