humbug n.

1751, in Oxford and Cambridge student slang, "a trick, jest, hoax, imposition, deception," a word of unknown origin; it also appeared simultaneously as a transitive verb, "deceive by false pretext." A vogue word of the early 1750s; its origin was a subject of much whimsical speculation even then. "[A]s with other and more recent words of similar introduction, the facts as to its origin appear to have been lost, even before the word became common enough to excite attention" [OED].

THERE is a word very much in vogue with the people of taste and fashion, which, though it has not even the penumbra of a meaning, yet makes up the sum total of the wit, sense, and judgment of the aforesaid people of taste and fashion. This word is HUMBUG. [The Student, vol. II no. 2, 1751]  

The meaning "spirit of deception or imposition; hollowness, sham" is from 1825.