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Old English into, originally in to. The word is a late Old English development to replace the fading dative case inflections that formerly distinguished, for instance, "in the house" from "into the house." To be into something, "be intensely involved in," first recorded 1969 in American English.
Doing the sex act; making love: Like Kirkland's book, Tharp's even includes a discussion of what Baryshnikov is like in the sack (1960s+)