I’ve done themed posts several times in the past, picking a yojijukugo or kotowaza based on the number of the post, or some event or holiday happening around the same time. This is the start of a new sub-project, in which I go through the 50-syllable set in order, from あ to わ (or even を, if I can find appropriate entries, perhaps using classical orthography… but ん is simply out, I’m afraid). For purposes of this ordered sub-project, I’ll be counting voiced and unvoiced syllables as a single unit, so for example a “ki” entry could start with き or ぎ; “fu” could be ふ, ぶ, or ぷ.
Literally: darkness – middle – imitation – rope / search
Alternately: Groping around in the darkness. Searching for something blind, without any leads. Trying various things out without any clues beyond what one learns through trial and error. Mosaku is “searching by hand,” and anchuu is “in the darkness.”
Notes: The third character may also be written as 摸 without any change in pronunciation or meaning. This seems to be the version used in China, as well.
The origin of this compound seems to be in a text called the Sui-Tang jiahua (『隋唐嘉話』), although casual searching reveals little further information not written in Chinese.