midi-
combining form
of medium or middle size, length, etc
⇒ 
midibus
Origin
C20: from mid-; on the model of maxi and minimidi-
                combining form
     -          medium-sized; of medium length表示“中型的”, “中长的”:
-       midibus midi-skirt. 
词源
from MIDDLE , on the pattern of maxi- and mini-.
1967 Word Study Dec. 3/2 In contrast to the miniskirt a skirt whose hemline comes as low as midcalf has been called a midiskirt. 
1968 Chicago Tribune 9 July 11. 1/7 A swaggering midi-coat that gives a new proportion to the trouser costume. 
1969 Sun 13 Mar. 8/2 The newsy midi hem-length (note midi, not maxi). 
1969 Internat. Herald Tribune (Paris) 6 Nov. 6/2Almost every designer is nervously groping for the right length; it's touch and go whether the average woman will go for the maxis and midis. 
1970 Daily Tel. 14 Jan. 15/3Hardy Amies proved yesterday..that the midi length only works if it is waisted or broken by a hip-belt. His spring collection was awash with midis. 
Ibid. 15/4Midi-dresses with big ballooning sleeves, hip sashes, polo necks. 
1971 Petticoat 17 July 23 Baggage and General midi bumper boots, {pstlg}4. 
1974 Country Life 14 Feb. 334/2 A slim-cut, midi-length trench . [ coat] 
ORIGIN: from mid  adjective middle  adjective maxi- mini-