midwinter (n.)

"middle; being the middle part or midst; being between, intermediate," Old English mid, midd from Proto-Germanic *medja- (source also of Old Norse miðr, Old Saxon middi, Old Frisian midde, Middle Dutch mydde, Old High German mitti, German mitte, Gothic midjis "mid, middle"), from PIE root *medhyo- "middle."

By late Middle English probably felt as a prefix only, and now surviving in English only as a prefix (mid-air, midstream, etc.). Prefixed to months, seasons, etc. from late Old English. As a preposition, "in the middle of, amid" (c. 1400) it is from in midde or a shortened form of amid (compare midshipman) and sometimes is written 'mid.

Old English winter (plural wintru), "the fourth and coldest season of the year, winter," from Proto-Germanic *wintruz "winter" (source also of Old Frisian, Dutch winter, Old Saxon, Old High German wintar, German winter, Danish and Swedish vinter, Gothic wintrus, Old Norse vetr "winter"), probably literally "the wet season," from PIE *wend-, nasalized form of root *wed- (1) "water; wet"). On another old guess, cognate with Gaulish vindo-, Old Irish find "white." The usual PIE word is *gheim-.

As an adjective in Old English. The Anglo-Saxons counted years in "winters," as in Old English ænetre "one-year-old;" and wintercearig, which might mean either "winter-sad" or "sad with years." Old Norse Vetrardag, first day of winter, was the Saturday that fell between Oct. 10 and 16.