forthyoudaoicibaDictYouDict[forth 词源字典]
forth: [OE] Forth can be traced back to the same Indo-European source, *pr, as produced English first, for, fore, foremost, former, from, and before. It formed the basis of a word *prto ‘forwards’, whose Germanic descendant *furtha gave German fort, Dutch voort, and English forth. Modern English forward(s) [OE], which has largely replaced forth in general use, was originally a compound formed from forth and -ward.

Other related forms include afford, which originally meant ‘accomplish, fulfil’, forthwith [14], at first literally ‘along with’, hence ‘at the same time as’ and ‘immediately’, and further.
=> afford, before, first, for, fore, former, from, further[forth etymology, forth origin, 英语词源]
forth (adv.)youdaoicibaDictYouDict
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.