"Truth goose" is a created phrase by Tim O'Brien that is partly American slang. It was used by the author of The Things They Carried, a work of fiction that is a collection of tales about his experiences in the Vietnam Conflict. From the author's reaction to the words of another soldier's words and his use of this phrase, the reader may infer that there was something so true about what the soldier said that the author has such a strong reaction to this surprising truth it is as if he had been "goosed," or poked between the buttocks.
O'Brien uses this phrase in his third story entitled "Spin" in which one of the soldiers went AWOL [left his troop without leave] and went to Danang where he stayed--"shacked up"--with a Red Cross Nurse. According to the soldier, she treated him wonderfully as he got "whatever he want[ed] whenever he want[ed] it." Nevertheless, he rejoined the troop one day. When the other soldiers asked him why he did this, the soldier told them, "All that peace, man, it felt so good it hurt. I want to hurt it back." The author, O'Brien, remarks,
Most of it he made up, I'm sure, but even so it gave me a quick truth goose.
The happiness that the soldier felt while he was with the nurse was intensely deep because of its contrast to the anxiety and pain that he had felt in the jungles of warfare. This joy and relief were sharply in contrast with the gut-wrenching fear of not knowing what was next, and the terror of walking across mine-fields, etc. Angered that he had endured such feelings, the soldier returns to war, hoping to end such terrible conflict of emotions in himself. Another wording of this interpretation is here on Enotes in the chapter summary: "The peace was hurting him and he wanted to hurt it back." Indeed, war does terrible things to soldiers.