When did people start using the corporate jargon "let's take it offline" (let's discuss that after this meeting in private)?
According to the Online Etymology Dictionary, the origin of online is from 1950:
online in ref. to computers, "directly connected to a peripheral device," is attested from 1950 (originally as on-line).
If the word offline in the jargon is the antonym of online as defined above, then the jargon cannot be interpreted literally. It is often used in meetings where people are all physically in the same room.