Origin and history of mainstream

early 13c., "notably large, bulky, or strong" (a sense now obsolete), from Old English mægen- "power, strength, force," used in compounds (such as mægensibb "great love," mægenbyrðen "heavy burden;" see main (n.)), probably also in part from or influenced by cognate Old Norse megenn (adj.) "strong, powerful, mighty."

Sense of "chief, principal, prime" is from c. 1400. That of "principal or chief in size or extent" is from 1590s. Main chance "opportunity of enriching oneself" is by 1570s, from the game of hazard. Main course in the meal sense attested from 1829. Main man "favorite male friend; hero" is by 1967, African-American vernacular.

Middle English strem "course of water, current of a stream, body of water flowing in a natural channel," from Old English stream, 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."

Boutkan writes, "The Gmc. insertion of the -t- in the cluster *sr is automatic," and compares Old English swester "sister" from PIE *swesr-.

From early 12c. as "anything issuing from a source and flowing continuously." Also sometimes in Old English and Middle English "the ocean, the sea," or a navigable channel of it; the meaning "steady current in the sea" (as in Gulf Stream) is recorded from late 14c., as is the sense of "steady current in a river."

The general sense of "continued course or current (of anything) moving in the same direction" is by 1580s. Stream of thought is from 1890 in psychology. Stream of consciousness in literary criticism is recorded by 1930, earlier in psychology (1855).