This is a military acronym, from the Vietnam war era. It stands for "DS" which is how military personnel would describe a bad situation, or poorly planned mission. It stands for "dog shit."
This phrase is derived using the phonetic alphabet (Military Alphabet). Instead of saying Dog Shit, you would say, Delta Sierra instead so everyone over the radio can understand and know what you mean.
By using the first letter of each word and converting that to the Military Alphabet you get the phrase "delta sierra."
D = Delta
S = Sierra
This mission is totally "delta sierra."