millstream (n.)

Middle English mille, "building fitted to grind grain," Old English mylen "a mill" (10c.), an early Germanic borrowing from Late Latin molina, molinum "mill" (source of French moulin, Spanish molino), originally fem. and neuter of molinus "pertaining to a mill," from Latin mola "mill, millstone," related to molere "to grind," from PIE root *mele- "to crush, grind." The -n- gradually was lost in English but survives in the surname Milner. Also from Late Latin molina, directly or indirectly, are German Mühle, Old Saxon mulin, Old Norse mylna, Danish mølle, Old Church Slavonic mulinu.

The meaning "mechanical device for grinding grain for food" is from 1550s. The broader sense of "machine for grinding or pulverizing any solid substance" is attested from 1670s. Other types of manufacturing machines driven by wind or water, that transform raw material by a process other than grinding began to be called mills by early 15c. Sense of "large building fitted with machinery for manufacturing" is from c. 1500. In old slang also "a typewriter" (1913); "a boxing match or other pugilistic bout" (1819).

Old English stream "a course of water," from Proto-Germanic *strauma- (source also of Old Saxon strom, Old Norse straumr, Danish strøm, Swedish ström, Norwegian straum, Old Frisian stram, Dutch stroom, Old High German stroum, German Strom "current, river"), from PIE root *sreu- "to flow."

From early 12c. as "anything issuing from a source and flowing continuously." Meaning "current in the sea" (as in Gulf Stream) is recorded from late 14c., as is the sense of "steady current in a river." Stream of consciousness in lit crit first recorded 1930, originally in psychology (1855). Stream of thought is from 1890.