Today's BWOTD has two meanings, the first is an abbreviation:
"I bought a new pair of wellies to garden in"
'Wellies' = Wellington Boots or tall Rubber Boots used when outside
The second meaning is a commonly used expression in the UK:
"I am going to really have to give it some welly"
'Welly' = really, really give it a go
So with this word, the spelling and meaning is all in the usage!