Funny English – Ricky Gervais Joke

In one of his comedy dramas, Ricky’s character told a joke:
Q – “What’s ET short for?”
A – “He’s only got little legs.”

Most people would assume “What’s ET short for?” means that ET is an abbreviation for what? (The answer would be “Extra Terrestrial”.) For example, one might say, “What’s CD short for?” The answer is “Compact Disc”.
This joke is a wordplay on the double meaning of “for”. The joke meaning is “for”=”why”, for example, “What did you come here for?”=”Why did you come here?”
So, it means “Why is ET short?”