forth (adv.) Look up forth at Dictionary.com
Old English forð "forward, onward, further; continually;" as a preposition, "during," perfective of fore, from Proto-Germanic *furtha- "forward" (cognates: Old Frisian, Old Saxon forth "forward, onward," Old Norse forð, Dutch voort, German fort), from extended form of PIE root *per- (1) "forward, through" (see per). The construction in and so forth was in Old English.