|1.||informal smooth or easy progress|
|2.||nautical Compare plane sailing sailing in a body of water that is unobstructed; clear sailing|
Easy going; straightforward, unobstructed progress. For example, The first few months were difficult, but I think it's plain sailing from here on. Alluding to navigating waters free of hazards, such as rocks or other obstructions, this term was transferred to other activities in the early 1800s.