REAL DEFINITION: It is the creamy peanut-butter like substance that is produced from anal sex with a partner who forgot to clear the shoot in advance. The "churning" of the "butter" creates a quacking sound. Hence: Duck Butter.
A mixture of sweat, dingleberries, baby powder, loose pubes, urine, and dirt that collects in the chode of a soldier after they've been in the field without a shower for more than a couple days. It can be scraped off with a fingernail, mixed with water and rubbed on the face and arms for insect repelent.
Matt, "I order you to take a shower when we get back to base. You fuckin' stink!"
Jeff, "I know. I'm churnin' some serious duck butter!"