n.1.One who is deficient in understanding; a witless person.
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in classic literature ?
"If you dare open that swine's mouth against me," shouted the woodman, "I'll crop your ears for you before the hangman has the doing of it, thou long-jawed lackbrain."
ALL those who had never heard of David Beckham will now know exactly who he is - the petulant lackbrain who scuppered his nation's World Cup chances by getting himself needlessly sent off against Argentina.