Hugooooo provided a verifiable example from 1968.
People have been described as being ‘well in’ (on good or close terms) with another person or group since 1781. But a more recent development, at least in British and Irish colloquial use, is being tracked by the OED, specifically describing a situation where one’s sexual or romantic advances to another person are very likely to be successful. In common with this new sense’s progenitor, very often one can be described as being well in with one’s intended target/sweetheart, but use of this sense may also be followed by there, as in our current earliest evidence, from 1997 (the unconventional spelling is characteristic of the Scottish author Irvine Welsh):
There’s you chattin up that Gillian… Yir well in their.
1997 Irvine Welsh State of the Party in S. Champion Disco Biscuits, p. 42
It seems likely that earlier evidence for this colloquialism exists: can you help?
Posted by OED_Editor on 2 October 2014 17.33