The word Tsawwassen means "Valley of the Sun". A legend says the town got it's name after a native american climbed over a large hill and looked down upon the valley that was in full sunlight and he exclaimed, "Tsawwassen!"
Also the present inhabitants of Tsawwassen usually just refer to it as T-town.
Also another note, this is referring to the write up above. Tsawwassen is not very flat at all, there is actually a giant hill in the middle of it, and this hill undulates. Actually Tsawwassen has many ascending and descending parts. The only flat part I can think of is where all the stores are. But when you get to the actual area where people live, there are hills everywhere.