It originated during british time. There was a british officer walking around at some kampung. And suddenly he came accross a long ditch.

Mesmerized by it, he asked a malay who was nearby. "Dear sir, what is this long thing called". The malay guy didn't understand, but looking at the british guy standing very near to it, the malay guy said "baik baik tuan, jatuh kang"

British guy asked "what?"
Malay repeat "jatuh kang!"
British "this long ...."
Malay guy "long? Jatuh kang!"
British "longkang?"
Malay guy "mampus la hang" n just walked away with a hand signal.

The british guy thought it was a sign for correct. And so, he went back n told his peers he saw a longkang. And so the word was spread.