where did the term "clean as a whistle" originate?

From PHRASES.ORG.UK

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.'" (From the "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997.)

: Another source states: "Robert Burns, in his poem, 'Earnest Cry,' used 'toom' ('empty') rather than 'clean' - 'Paint Scotland greetan owre her thrissle; Her mutchkin stoup as toom's a whissle' - and other writers have had the whistle clear, dry, pure or other adjective. The basic intent, however, is to indicate that, for a sweet, pure sound from a whistle or reed, the tube must be clean and dry." (From "Heavens to Betsy & Other Curious Sayings" by Charles Earle Funk, Harper & Row, New York, 1955.)

: And a third: ".As every old-timer can tell you, a good whistle made from a reed or a piece of wood emits a clear tone - but it is easily damaged. Even small particles of debris, or a few drops of moisture will change the sound of a handmade instrument. In order to emit the pure notes intended by its maker, a whistle has to be absolutely clean. Anything or anyone as clean as a brand-new whistle or as clear as its sound is bound to be good. All of which means that an organization or person called as 'clean as a whistle' has been judged to be guiltless or flawless." (From "Why You Say It" by Webb Garrison, Rutledge Hill Press, Nashville, Tenn., 1992.)

Alternatively the origin may be the clean appearance of a just carved wooden whistle. Personally, I think it may well relate to locomotives where the brass, especially the whistle was always bright and gleaming.

I don't know, but found this:

The origin of the phrase "clean as a whistle" is?

: 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.'" (From the "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997.)

: Another source states: "Robert Burns, in his poem, 'Earnest Cry,' used 'toom' ('empty') rather than 'clean' - 'Paint Scotland greetan owre her thrissle; Her mutchkin stoup as toom's a whissle' - and other writers have had the whistle clear, dry, pure or other adjective. The basic intent, however, is to indicate that, for a sweet, pure sound from a whistle or reed, the tube must be clean and dry." (From "Heavens to Betsy & Other Curious Sayings" by Charles Earle Funk, Harper & Row, New York, 1955.)

: And a third: ".As every old-timer can tell you, a good whistle made from a reed or a piece of wood emits a clear tone - but it is easily damaged. Even small particles of debris, or a few drops of moisture will change the sound of a handmade instrument. In order to emit the pure notes intended by its maker, a whistle has to be absolutely clean. Anything or anyone as clean as a brand-new whistle or as clear as its sound is bound to be good. All of which means that an organization or person called as 'clean as a whistle' has been judged to be guiltless or flawless." (From "Why You Say It" by Webb Garrison, Rutledge Hill Press, Nashville, Tenn., 1992.)

Alternatively the origin may be the clean appearance of a just carved wooden whistle. Personally, I think it may well relate to locomotives where the brass, especially the whistle was always bright and gleaming.

Clean As A Whistle

The phrase "clean as a whistle' means exceptionally clean or smooth.

The most popular well know version is that this phrase appeared at the beginning of the 19th Century, referring to the whistling noise made as a sword tears swiftly through the air to , for example ;cut through something in a single stroke.

However, having said that I remember reading somewhere that it actually referred to the policeman's shiny whistle which was always polished clean especially in the past.

Incidentally according to the “Morris Dictionary of Word and Phrase Origins” by William and Mary Morris(Harper Collins, New York, 1977, 1988). ROGER -- "in the meaning of 'Yes, O.K., I understand you -- is voice code for the letter R. It is part of the 'Able, Baker, Charlie' code known and used by all radiophone operators in the services in the 40's - 50's. From the earliest days of wireless communication, the Morse code letter R (dit-dah-dit) has been used to indicate 'O.K. -- understood.' So 'Roger' was the logical voice-phone equivalent." Also from “I Hear America Talking” by Stuart Berg Flexner (Von Nostrand Reinhold Co., New York, 1976).“Roger! A code word used by pilots to mean ‘your message received and understood’ in response to radio communications; later it came into general use to mean ‘all right, OK.’ Roger was the radio communications morse code word for the letter R, which in this case represented the word ‘received.’ ‘Roger Wilco’ was the reply to ‘Roger’ from the original transmitter of the radio message, meaning ‘I have received your message that you have received my message and am signing off.” Wilco implies "I will comply"