Why is David Tennant's nickname David Ten-Inch?

Answer 1

Billie Piper accidentally mispronounced his name once as 'teninch' instead of 'tennant'; she thought it sounded funny because it immediately brought to mind the dirty thought that what if he had a ten-inch.

She repeated it deliberately to him as a joke; someone else hear it and thought it was funny and the joke spread. It's now just a joke that girls ask him: "are you a ten-ant or a ten-inch?".

Well if his manhood is larger than an ant, than the answer is ten-inch.

But really, it all just started with Billie Piper accidentally saying his name wrong; it's in her auto-biography.

Answer 2

Billie Piper came up with it because she thought it was funny.