1. an imaginary creature resembling an elephant;
also, an elephant ...1926 colloq.
2. (potentially offensive) a fat or ungainly person ...1993 colloq.
apparently from an arbitrary first element + lump (n. a compact mass of no particular shape, often with an implication of excessive size)
FIRST DOCUMENTED USE
1926 - see EXAMPLE below
"...And then he had a Clever Idea. He would go up very quietly to the Six Pine Trees now, peep very cautiously into the Trap, and see if there was a Heffalump there. And if there was, he would go back to bed..."
From: Winnie the Pooh
- Alan Alexander Milne