astride (adv.)

prefix or inseparable particle, a conglomerate of various Germanic and Latin elements.

In words derived from Old English, it commonly represents Old English an "on, in, into" (see on (prep.)), as in alive, above, asleep, aback, abroad, afoot, ashore, ahead, abed, aside, obsolete arank "in rank and file," etc., forming adjectives and adverbs from nouns, with the notion "in, at; engaged in." In this use it is identical to a (2).

It also can represent Middle English of (prep.) "off, from," as in anew, afresh, akin, abreast. Or it can be a reduced form of the Old English past participle prefix ge-, as in aware.

Or it can be the Old English intensive a-, originally ar- (cognate with German er- and probably implying originally "motion away from"), as in abide, arise, awake, ashamed, marking a verb as momentary, a single event. Such words sometimes were refashioned in early modern English as though the prefix were Latin (accursed, allay, affright are examples).

In words from Romanic languages, often it represents reduced forms of Latin ad "to, toward; for" (see ad-), or ab "from, away, off" (see ab-); both of which by about 7c. had been reduced to a in the ancestor of Old French. In a few cases it represents Latin ex.

[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose. [OED]

"a step in walking," especially a long one, from Old English stride "a stride, a step," from the root of stride (v.). Compare Dutch strijd, Old High German strit, German Streit "fight, contention, combat," Swedish and Danish strid "combat, contention." From c. 1300 as a measure of distance roughly the length of a stride. Figurative meaning "advance rapidly, make progress" is from c. 1600. Of animals (especially horses) from early 17c. To take (something) in stride (1832), i.e. "without change of gait," originally is of horses leaping hedges in the hunting-field; figurative sense attested from 1902. To hit (one's) stride is from horse-racing. Jazz music stride tempo is attested from 1938. Meaning "a standing with the legs apart, a straddle" is from 1590s.