thenceforth (adv.)

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.

"from that place," originally usually implying motion, late 13c., thennes, with adverbial genitive -s + thenne "from that place," from Old English þanone, þanon "from that place." This is reconstructed to be from Proto-Germanic *thanana (source also of Old Saxon thanana, Old Norse þana, Old Frisian thana, Old High German danana, German von dannen), which is related obscurely to the root of then, and ultimately from PIE demonstrative base *to- (see the).

Written with -c- to indicate a voiceless "s" sound. The meaning "from that time" is from late 14c.; the sense of "for that reason" ("from that source") is from 1650s. From thence (late 14c.) is redundant.

Middle English also had thennesward (c. 1400), Old English þanenweard "from that place."