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Saying you're down to do something means the same thing as saying you're up for it.
(discuss.tchncs.de)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
I thought it is short for "I am putting myself down for that" or "put me down for that". As in, putting yourself down on a list for attending an event.