Into

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Old English intō (see in, to).

wiktionary

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From Middle English in-to, from Old English intō, equivalent to in +‎ to. Cognate with Scots intae.

etymonline

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into (prep.)

Old English into "into, to, against, in," originally in to. It emerged in late Old English to do the work of the dative case inflections, then fading, that formerly distinguished, for instance, the notion of "in the house" from that of "into the house." Compare onto, unto. To be into (something) "be intensely involved in or devoted to" recorded by 1967 in American English youth slang.