Q:

What is the origin of the term _akachinki_, borrowed into Korean as _akkajinki_? (Meaning "Mercurochrome" or "Merbromin")

There is a topical antiseptic called Mercurochrome or Merbromin.  According to the Korean Wikipedia article, an alternative name for this in Korean is [아]【a】[까]【kka】[징]【jin】[끼]【kki】 (possibly also romanized as akkajinki), which was borrowed from Japanese 赤【あか】チンキ.
What is the origin of this Japanese term akachinki?

A:

The more formal Japanese term for generic name Merbromin is メ【me】ル【ru】ブ【bu】ロ【ro】ミ【mi】ン【n】, and the term for brand name Mercurochrome is マ【ma】ー【a】キュ【kyu】ロ【ro】ク【ku】ロ【ro】ム【mu】.  This topical antiseptic also goes by the informal name [赤]【aka】チ【chi】ン【n】, or older [赤]【aka】チ【chi】ン【n】キ【ki】.

The [赤]【aka】 part simply expresses the sense of red, as the medicine is indeed red in color:

The チ【chi】ン【n】 part is from older チ【chi】ン【n】キ【ki】, which in turn is an abbreviation of チ【chi】ン【n】キ【ki】テュ【tyu】ー【u】ル【ru】, a borrowing from Dutch tinktuur, cognate with English tincture (i.e.,  a solution of a medicinal substance in a liquid, usually alcohol).

The Japanese Wikipedia article suggests that the term arose from a slang use of チ【chi】ン【n】キ【ki】 to refer more specifically to ヨ【yo】ー【o】ド【do】チ【chi】ン【n】キ【ki】, or a tincture of iodine.  Since iodine tinctures are more yellowy-brown in color, the [赤]【aka】 was added as a prefix to differentiate.

