Wellies are long rubber boots which you wear to keep your feet dry. wellies a wellington (=kind of boot)
{i} Wellington boot, style of high waterproof boot that reaches to the knees and is cut lower behind the knee (generally used in plural form: Wellingtons, also spelled wellie); power, vigor; acceleration (American Slang); hard kick (American Slang)
{i} Wellington boot, style of high waterproof boot that reaches to the knees and is cut lower behind the knee (generally used in plural form: Wellingtons, also spelled welly)