colloquial abbreviation of midshipman, by 1818. As "loose, long type of women's blouse," 1911, from resemblance to shirts worn by midshipmen.
Entries linking to middy
midshipman n.
naval officer, c. 1600, originally so called because he was stationed amidships when on duty (see amid). Midships as short for amidships is by 1620s. Midship as "the middle of a ship or boat" is from 1550s.