LOL Yes a welly, short for Wellington Boot, is a rain boot or galoshes. They used to be often called "rubbers"...but that has fallen out of use. I wonder why...
In fact the drink and wet baby Snookums, is called Baby Wellyboot in the UK. Which makes more sense since she has a BOOT on her flank! They could have called her rain boot or something couldn't they?
That boot is very cute!