(Secchin de manjuu; “A bun in the outhouse”)
Hiding away in order to enjoy something all by oneself, like squirreling oneself away in the toilet to have a snack (or go online with a smartphone, these days). Other, less common uses point to being so hungry that you don’t care where you eat, or being unable to choose where you eat (and therefore unable to enjoy the food).
This is just two nouns connected by a particle. We begin with 雪隠 (secchin), an outhouse, marked as the place of action by the particle で (de). Next comes the noun 饅頭 (manjuu), a steamed yeast bun still popular as a snack or dessert in Japan. The actual verb is elided, but we can assume that the action is eating.
Looking at the characters used to write 雪隠, one might expect it to be pronounced setsuin – and originally it was, but over time the pronunciation shifted to secchin. Further drift has also produced senchi as an accepted pronunciation. The term’s origins are unclear, other than that it comes from China, but generally involve the outhouse belonging to a Buddhist temple.
(Otouto ni katta koto wo shirareru mae ni, ani wa uraniwa de kossori to secchin de manjuu wo kuu you ni manga wo yonda.)
[Before his younger brother realized that he had bought it, the boy stealthily read the manga in the back yard as if secretly enjoying a treat.]