Therein

wiktionary

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From Middle English theryn, ther-inne, þer-in, from Old English þǣrin, þǣrinne, from Proto-West Germanic *þārin, *þārinna, from Proto-Germanic *þarin, *þarinna, equivalent to there +‎ in. Cognate with Saterland Frisian deerin(“therein”), West Frisian dêryn(“therein”), Dutch daarin(“therein”), German Low German daarin(“therein”), German darin, drin(“therein”), Danish deri(“therein”), Swedish däri(“therein”), Icelandic þar í(“therein”).

etymonline

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therein (adv.)

"in that place, time, or thing," Old English þærin; see there + in. Similar formation in German darin.