I've known of the saying "I'm not a nitpicker nor a nitpicker's son, but I'll pick your nits 'til the nitpicker comes" since I was young, and so have the local county librarians. Where does it originate, or what is its background? My diligent attempts searching for an answer online have been futile, and the local county librarians are stumped, too.
This seems to be a bowdlerization of a well-known tongue-twister:
Repeat 3 times: “I’m not a pheasant plucker, I’m a pheasant plucker’s son, and I’m only plucking pheasants till the pheasant plucker comes”.
Alan Dearlig (1980). The Youth Games Book, p. 105. Edinburgh: Intermediate Treatment Resource Centre.
(To “bowdlerize” is “to modify words or passages considered indelicate or offensive” (OED), in this case to avoid the accidental (or accidentally on purpose) metathesis of “pheasant plucker” to “pleasant fucker”.)
The tongue-twister seems to have originated in the UK in the early 1970s. Other early appearances in print are:
Earlier this year, I cut a record which could be loosely described as a folk or country ballad, “I’m Not a Pheasant Plucker, I’m a Pheasant Plucker’s Son.” The record was semi-banned by the B.B.C. I think they were a bit concerned about the disc jockeys getting their tongues in a twist.
Bill Maynard (1975). The Yo-Yo Man, p. 62. London: Golden Eagle.
He [John Spencer] used to† make up‡ little poems. ‘I’m not a pheasant plucker; I’m a pheasant plucker’s son; When I’m not plucking pheasant…’ and so on.
Gareth Edwards (1978). Gareth, p. 146. London: Stanley Paul.
† Edwards is not very specific about dates, but a plausible way to read him places the anecdote during the 1971 Lions tour. ‡ We’re not obliged to take this claim of authorship seriously: it’s normal for people to claim, or allow it to be understood, that they came up with jokes or stories, when in fact they are passing on folklore.
I’m not a pheasant plucker
I’m the pheasant plucker’s mate
I’m only plucking pheasants
’Cos the pheasant plucker’s late!
John Dunn (1982). John Dunn’s Curious Collection, p. 25. London: Frederick Muller.
Before incorporation into the tongue-twister, “pheasant plucker” was rhyming slang:
pheasant plucker Pleasant f——er. 20C. Primarily a Spoonerism, it is, perhaps, not fully admitted to the status of rhyming slang, but in its altered form—FEATHER PLUCKER (q.v.) it qualifies. It is generally (if not always) intended to be reversed in meaning as well as its form: ‘You are [or he is] a pheasant plucker, I must say!’ means—you are, or he is, a very unpleasant person.
Julian Franklyn (1960). A Dictionary of Rhyming Slang, p. 161. London: Routledge and Paul.
Originally published in 1899 it's an old English rhyme which describes an actual trade from that era, the "Pheasant Plucker" was later "covered" by a variety of bands and artists who were inspired by this tongue twister and include The Wurzels, Irish Rovers, Seamus Moore and Bill Maynard etc. Original wording as follows: I’m not the pheasant plucker, I’m the pheasant plucker’s mate, And I’m only plucking pheasants ’cause the pheasant plucker’s late. I’m not the pheasant plucker, I’m the pheasant plucker’s son, And I’m only plucking pheasants till the pheasant pluckers come. (alt ending: I'm only plucking pheasants till the pheasant pluckings done).
Try saying 3 times quickly, the obvious errors will creep in :D