outpouring (n.)

in Old English a common prefix with nouns, adjectives, adverbs, and verbs, "out, outward, outer; forth, away," from out (adv.). The use was even more common in Middle English, and also with the senses "outer, outside, on the outside, from without, external, externally; apart; greatly, extremely; completely, thoroughly, to completion." Other senses of out that extended into the use as a prefix include "beyond the surface or limits; to the utmost degree; to an explicit resolution."

In composition out has either its ordinary adverbial sense, as in outcast, outcome, outlook, etc., or a prepositional force, as in outdoors, or forms transitive verbs denoting a going beyond or surpassing of the object of the verb, in doing the act expressed by the word to which it is prefixed, as in outrun, outshine, outvenom, etc. In the last use especially out may be used with almost any noun or verb. [Century Dictionary]

"to cause (liquid or granular substance) to flow or stream either out of a vessel or into one," c. 1300, of unknown origin. Not in Old English; perhaps from Old French (Flanders dialect) purer "to sift (grain), pour out (water)," from Latin purare "to purify," from purus "pure" (see pure). Replaced Old English geotan. Intransitive sense of "to flow, issue forth in a stream" is from 1530s. Related: Poured; pouring; pourable. As a noun from 1790, "a pouring stream."