Apparently, there was a time when all holes, or openings, in the kimono were called "8 Holes". Not exactly sure why but maybe it was because there are 8 all together.
Below is a list of the names of the kimono openings. (From here.)
- 長着の穴について - Nagagi no ana ni tsuite - "Long Clothing Openings"１． 首を通す穴 - Kubi o toosu ana - Head Opening
２． 足を通す穴 - Ashi o toosu ana - Feet Opening
３． 右手を通す穴（※右の袖口）Migite o toose ana (migi no sodeguchi) - Right hand opening
４． 右の振り八つ口 - Migi no furiyatsuguchi - Right sleeve edge opening
５． 右の身八つ口 - Migi no miyatsuguchi - Right body opening (armpit opening)
６． 左手を通す穴（※左の袖口）- Hidarite o toose ana (hidari no sodeguchi) - Left hand opening
７． 左の振り八つ口 - Hidari no furiyatsuguchi - Left sleeve edge opening
８． 左の身八つ口 - Hidari no miyatsuguchi - Left body opening (armpit opening)
Over the course of time, I'm assuming that the "8 hole" bit was dropped out of some of terms. For example, the opening on the edge of the sleeve is called "furiguchi 振り口" in some sources and "furimiyatsuguchi 振り八つ口" on others. "Sodeguchi" may have been "sodemiyatsuguchi" at some point as well. But they are pretty long terms; it's not wonder why the terms were shortened.
Perhaps the miyatsuguchi kept it's full name because otherwise it would have been just "body hole" and then it wouldn't be clear whether it was the head opening or not.