There are few things more dreaded in US airports than a long, snaking security line. But, if you see the letters "SSSS" printed on the bottom of your boarding pass, expect to spend some extra time with the fine people at Transportation Security Administration (TSA).

That's because the passengers whose boarding passes bear these four letters have been pre-selected for additional security screening.

"SSSS stands for Secondary Security Screening Selection and it appears on a passenger's boarding pass when they have been selected by TSA's Secure Flight system for enhanced security screening," a TSA spokesman told Business Insider in a statement.

A Transportation Security Administration agent dons rubber gloves at a security checkpoint at Washington Reagan National Airport. Reuters

According to the TSA, the meaning of "SSSS" has been public knowledge for years. However, the average traveller would be hard pressed to find reliable information on the mysterious four-letter designation without approaching the TSA.

In that same statement, the TSA explained the system used to select passengers for secondary screening.