a Wellington boot (rubberised or plastic waterproof footware). In the 1970s, it began to be used in phrases where 'boot' occurred, e.g. 'The welly's on the other foot now'; 'he's getting too big for his wellies'. Perhaps started by The Great Northern Welly Boot Show put on by Billy Connolly at the Edinburgh Festival in the early 1970s. 'Well boot' is also a variant UK, 1961.

power, energy, especially when harnessed as acceleration UK, 1980.► give it some wellie; give it some wellyto vigorously attack someone or something; hence, to put all your effort into something. After WELLIE (Wellington boot), hence a play on STICK THE BOOT IN UK, 1990