1In the position of having established a good relationship with someone; on good terms.
2In a situation or position where one's sexual or romantic advances are very likely to be successful.
With a very good chance; with favourable odds.
4Australian So as to be well established; in favourable or comfortable circumstances; specifically (Australian) = "well off".
Late 18th century; earliest use found in Peter Beckford (1740–1811), dog breeder and writer on hunting. From well + in.