Flea is a term used in land-based casinos to refer to an annoying person at a table.
Most often, the term is used when there’s an entitled loudmouth who annoys other players as well as casino staff by expecting complimentary rewards, such as RFB and comp points, while only placing small wagers.
As you may know, comp rewards are most often reserved for players who bet big or those who regularly play in the same casino. Sometimes, casino staff even gives comp rewards to players who are having a rough night and are on a losing streak.
However, casino staff prefers doing that on their own accord, and they are the ones who determine who will receive comp rewards. Because of that, someone who continuously asks for such awards is only bothering them and potentially other players as well, disrupting the gameplay with their requests.
Keep in mind that while the term is somewhat justified, it is also derogatory. For that reason, it would be best to avoid using it so that you don’t end up offending someone.