I know'd this was a whapper of a lie, as soon as I heard it.
And turning to Tengelyi, he added, "That boy Imri is a whapper, sir; but the other chap's a scurvy rat!"
1785, formed as if from whop (v.) "to beat, overcome." Meaning "big lie" is recorded first in 1791. Whopping "large, big, impressive" is attested by 1620s.
[1785+; fr whop]