inroad n.

element meaning "into, in, on, upon" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "in," from PIE root *en "in."

In Old French (and hence in Middle English) this often became en-, which in English had a strong tendency to revert to Latin in-, but not always, which accounts for pairs such as enquire/inquire. There was a native form, which in West Saxon usually appeared as on- (as in Old English onliehtan "to enlighten"), and some of those verbs survived into Middle English (such as inwrite "to inscribe"), but all now seem to be extinct.

Not related to in- (1) "not," which also was a common prefix in Latin, causing confusion: to the Romans impressus could mean "pressed" or "unpressed;" inaudire meant "to hear," but inauditus meant "unheard of;" in Late Latin investigabilis could mean "that may be searched into" or "that cannot be searched into." Latin invocatus was "uncalled, uninvited," but invocare was "to call, appeal to."

The trouble has continued in English; the hesitation over what is meant by inflammable being a commonly cited example. Implume (1610s) meant "to feather," but implumed (c. 1600) meant "unfeathered." Impliable can mean "capable of being implied" (1865) or "inflexible" (1734). Impartible in 17c. could mean "incapable of being divided" or "capable of being imparted." Impassionate can be "free from passion" or it can mean "strongly stirred by passion." Inanimate (adj.) is "lifeless," but Donne uses inanimate (v.) to mean "infuse with life or vigor." Irruption is "a breaking in," but irruptible is "unbreakable."

In addition to improve "use to one's profit," Middle English also had a verb improve meaning "to disprove" (15c.). To inculpate is "to accuse," but inculpable means "not culpable, free from blame." Infestive has meant "troublesome, annoying" (1560s, from infest) and "not festive" (1620s). In Middle English inflexible could mean "incapable of being bent" or "capable of being swayed or moved." In 17c., informed could mean "current in information," formed, animated," or "unformed, formless" ("This was an awkward use" [OED]). Inhabited has meant "dwelt in" (1560s) and "uninhabited" (1610s); inhabitable likewise has been used on opposite senses, a confusion that goes back to Late Latin.

Middle English rode, from Old English rad "riding expedition, journey, hostile incursion," from Proto-Germanic *raido (source also of Old Frisian red "ride," Old Saxon reda, Middle Dutch rede, Old High German reita "foray, raid"), from PIE *reidh- "to ride" (see ride (v.)). Also related to raid (n.).

In Middle English it was still, "a riding, a journey on horseback; a mounted raid;" the sense of "an open passage or way for traveling between two places" is recorded from 1590s, and the older senses now are obsolete. "The late appearance of this sense makes its development from sense 1 somewhat obscure," according to OED, which however finds similar evolutions in Flemish and Frisian words. The modern spelling was established 18c.  

The meaning "narrow stretch of sheltered water near shore where ships can lie at anchor" is from early 14c. (as in Virginia's Hampton Roads). In late 19c. U.S. use it is often short for railroad.

On the road "traveling" is from 1640s. Road test (n.) of a vehicle's performance is by 1906; as a verb from 1937. Road hog "one who is objectionable on the road" [OED] is attested from 1886; road rage is by 1988. Road map is from 1786; road trip is by 1950, originally of baseball teams. Old English had radwerig "weary of traveling."

"take part in a raid, make a hostile attack upon," 1864, from raid (n.). Related: Raided; raiding (by 1826 as a verbal noun). Also see raider.