YGBSM

From: http://www.militaryhistoryonline.com/vietnam/airpower/wildweasel.aspx

The new Weasel crews were shocked to hear of their new mission, which was nothing short of a suicide mission. One of the new weasels, EWO Captain Jack Donovan, summed up the feelings of all the Weasels about their new mission, when he said, " You want me to fly in the back of a little tiny fighter aircraft with a crazy fighter pilot who thinks he's invincible, home in on a SAM site in North Vietnam, and shoot it before it shoots me, you gotta be shittin me!" Despite the apprehension, the Weasel crews went forward with their suicide mission, and in early November 1965, they deployed to Korat AFB in Thailand to begin their Wild Weasel missions.

From that day on, the unofficial motto of the Wild Weasels became "You gotta be shittin' me", abbreviated as YGBSM.

Wild Weasels were a special group of airplanes tasked with SEAD (Supression of Enemy Air Defences) missions which involved "attracting" a SAM site's "attention" (which involved having a SAM fired at you) in order to have it disclose it's location, and destroy the SAM site and somehow survive to fight another day. Sounds almost as bad as those tunnel rats.