(Of a heated liquid) simmering strongly and about to boil.
(colloquial) About to happen; imminent.
(Of a heated liquid) boiling.
(colloquial) continuing; active; in a state of activity or development.
trying to keep the deal on the boil
(colloquial) Going smoothly; working well.
(sports, colloquial) Playing exceptionally well.
(sports, rugby, colloquial) (Of a player taking a pass) running at full speed towards the opposition.
Source: en.wiktionary.org