Old English forð "forward, onward, farther; continually;" as a preposition, "during," perfective of fore, from Proto-Germanic *furtha- "forward" (source also of Old Frisian, Old Saxon forth "forward, onward," Old Norse forð, Dutch voort, German fort), from extended form of PIE root *per- (1) "forward." The construction in and so forth was in Old English.
word-forming element in Old English and early Middle English, from old English riht "just, good, fair; proper, fitting; straight, not bent, direct, erect," which was used as the second element in compounds. See right (adj.1). Surviving in downright, forthright, etc.
c. 1200, "straight down, right down, perpendicularly," from down (adv.) + -right. The meaning "thoroughly, completely, utterly," often merely emphatic, is attested from c. 1300. As an adjective, "complete, absolute," from 1560s. Old English had dunrihte "downwards." The inverted form right-down is attested 17c.
<a href="https://www.etymonline.com/word/forthright">Etymology of forthright by etymonline</a>
Harper, D. (n.d.). Etymology of forthright. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/forthright