Every time I hear this idiom, I cogitate to no avail as to its sense. Why is it a whistle, and not a lantern, or an axe?
Etymonline indicates the first appearance of this phrase as 1878.
Take Our Word For It discusses a possible 1786 origin from a Scottish poet:
Robert "Rabbie" Burns (in his Author's Earnest Cry, 1786) provides us the first use of anything resembling the phrase clean as a whistle in writing: "Her mutchkin stowp as toom’s a whissle"... this meant "Her pint bucket is as empty as a whistle". ...we conjecture that Rabbie was familiar with this instrument, the implication being that if a whistle is not clear of obstruction inside, then it will not play properly.
The authors also connect "clear" (as in the pure sound of a whistle) to "clean":
The Phrase Finder summarizes several origin theories, including:
- the old simile describes the whistling sound of a sword as it swishes through the air to decapitate someone, and an early 19th century quotation does suggest this connection: 'A first rate shot.(his) head taken off as clean as a whistle.' (Encyclopedia of Word and Phrase Origins)
- Robert Burns, in his poem, 'Earnest Cry,' used 'toom' ('empty') rather than 'clean' ...other writers have had the whistle clear, dry, pure or other adjective. ...for a sweet, pure sound from a whistle or reed, the tube must be clean and dry. (Heavens to Betsy & Other Curious Sayings)
- Anything or anyone as clean as a brand-new whistle or as clear as its sound is bound to be good. ... an organization or person called as 'clean as a whistle' has been judged to be guiltless or flawless (Why You Say It)
The Word Detective further suggests:
The phrase actually has two meanings: "clean or pure" and "absolutely, completely." "Utterly or completely" is the original 18th century meaning -- a roof blown off in a tornado might be said to have been torn off "clean as a whistle," leaving no remnants. The "pure or unsullied" meaning ("Wash that deck until it's clean as a whistle, sailor") came later ...Christine Ammer, in her book "Have A Nice Day -- No Problem, A Dictionary of Cliches," points to the phrase "clear as a whistle," very common in the 18th century. ..."clear as a whistle" came to mean "unmistakable" or "unambiguous." ...the subsequent drift of "clean" in the phrase to mean "pure" is what has led to folks like you wondering "what's so clean about whistles?"
If such a change has taken place, it could explain why a phrase that can be connected to complete, obvious decapitation can also mean innocence or cleanliness.
The books I consulted disagree about the meaning and provenance of this phrase. At one extreme is this simple entry from Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995):
clean as a whistle (or a hound's tooth) adj phr first form by 1828, second by 1940s Perfectly clean
But Christine Ammer, American Heritage Dictionary of Idioms (1997) has a different take on the term:
clean as a whistle Completely, entirely, thoroughly, as in He chopped off the branch, clean as a whistle. The allusion in this simile is unclear. It my have been a replacement for the 18th-century clear as a whistle, which alluded to the pure, clean sound of a whistle (it has few overtones). However, it was adopted to describe something thoroughly done. [Early 1800s]
And Robert Hendrickson, Facts on File Encyclopedia of Word and Phrase Origins (1997) has this meditative entry:
clean as a whistle. One possibility is that the old simile describes the whistling sound of a sword as it swishes through the air to decapitate someone, and an early 19th-century quotation does suggest this connection: "A first rate shot...[his] head taken off as clean as a whistle." The expression is proverbial, at least since the 18th century, when Robert Burns used a variation on it. More likely the basic idea suggests the clear, pure sound a whistle makes, or the slippery smooth surface of a willow stick debarked to make a whistle. But there is also a chance that the phrase may have originally been as clean as a whittle, referring to a piece of smooth wood after it is whittled.
And going much farther back, we have John Ker, The Archaiology of Popular English Phrases and Nursery Rhymes (1834), offering this unusual analysis:
As CLEAN AS A WHISTLE; as in the phrase, "it was done as clean as a whistle:" and in the sense of, the act in question having been performed cleanly, neatly, suddenly, and without bustle. Als glij in haest er huij (wei) stil; q.e. as rapidly and imperceptibly (stilly) as whey separates from the rest of the substance (the curd); and what process can take place with greater quickness, silence, and requisite efficiency than that of the separation of whey (serum) from the curd (coagulum), throughout which, the instant before, it had been homogeneously distributed.
Even earlier, William Carr, The Dialect of Craven, in the West-Riding of the County of York (1828) has this:
"As clean as a whistle," a proverbial simile, signifying completely, entirely ; as, "I've lost my knife as clean as a whistle;" but I know not the propriety of this simile.
The earliest Google Books match for the phrase, however, is from Joseph Reed, The Register-Office: A Farce of Two Acts (1761):
Gulwell. Your Ladyship's most devoted—Of great Inconvenience to my Family Affairs to have Richard's Place unsupplied!—In Faith I believe her Layship!—So Dick is unshipp'd and the Bond not worth a farthing!—I have lost the five hundred Pounds, as clean as a Whistle!—He gave me such Assurances of her Ladyship's Regard, that I thought the Money as safe, as if I had it in my Pocket—Who's here?—one of my party-colour'd Customers!—Oh! 'tis Lady Vixen's Livery!
Joining clean as a hound's tooth and clean as a penny (which Ker cites) as expressions allied to clean as a whistle is clean as a button-stick, which Eric Partridge, in A Dictionary of Slang and Unconventional English, fifth edition (1961), defines as "(of a soldier) smart in appearance ... A button-stick was a device for polishing buttons."
Clean as a whistle has a nearly universal association with cuts. Clean cuts are not ragged but are smooth with crisp sharp edges. The author of Whittlin' Whistles describes how to make a slip-bark whistle by sliding the bark off a branch, carving it and sliding it back into the bark, and suggests that "clean as a whistle" relates to the smooth cuts, without which, he says, the whistle will not work.
Perhaps taken for "as clear as a whistle" (the expression has existed) ; either because the sound is clear, sharp, or because, of course, it does not work if obstructed.
Because a whistle will not blow if it is wet. The little ball in the whistle will still roll around, but the sound will be muffled severely.
The Free Dictionary has a comment to this idiom. They say the idiom is unclear. I have the impression that it is rather dated and an empty shell. The saying may be used but it does not convey a clear picture or a clear idea. Empty words. http://idioms.thefreedictionary.com/clean+as+a+whistle