The phrase to be part and parcel of means to be an essential feature or element of.
The primary meaning of the noun parcel is small part of a whole.
Via Anglo-Norman forms such as parcele and parcell, and Old and Middle French parcelle, this noun is from an unattested post-classical Latin particella, part, portion, alteration of classical Latin particula, denoting a small part, a little bit, a particle, diminutive of pars/part-, part, piece, portion, share, etc.
The nouns parcel and part were originally synonyms. For instance, in the Early Version (around 1382) of the Wycliffe Bible, the Book of Ecclesiasticus, 14:14, is:
Be thou not bigilid fro the good day, and the parcel of the goode day passe thee not.
Whereas, in the Later Version (1395), the verse is:
Be thou not disseyued of a good dai, and a litil part of a good day passe not thee.
In the King James Version (1611), it is:
Defraud not thy selfe of the good day and let not the part of a good desire ouerpasse thee.
Originally a legal formula used in defining ownership, the contents of estates, etc., part and parcel was merely emphatic, the second noun reinforcing the first, as is often the case in the jargon of legal documents. Both nouns meant an integral portion of something and were used to emphasise inclusion in the whole rather than partitive character. One of the early instances of the phrase is in the following passage from an Article promulgated in 1463 by the City of London:
Be it remembred that by Mathewe Philipp Maire Aldremen and Co’es of the Citee of London in theire Comune Consell holdene in the Yeldehall of the saide Citee the xxx day of Decembre the yeere of the reign of Kyng Edwarde the iiijᵗʰ after the conquest the iijᵈᵉ At the request praier and desire of the weldisposed blessed and devote woman Dame Agnes ‘Foster’ […] it is ordeigned that the Newark [= new work] now late edified by the saide Dame Agnes for thenlargyng of the Prysone of Ludgate aforesaid frome hensfourth be hadde, repute, and takene as a parte and parcell of the saide prysone of Ludgate.
The phrase was also part or parcel, for instance after any. The Rolls of Parliament (Rotuli parliamentorum) for the year 1451 contain the following:
Provided also, that the seide Petition, Acte, or Ordenaunce of Resumption, extende not nor be prejudiciall nor hurte unto the Abbot and Covent of the Monasterie of Seint Germayn of Selby, in the Diocise of York, nor to theire successours, of or for any Graunte or Grauntes, or Ordenaunce of exoneration or discharge, in any wyse made or graunted by us by oure Letters Patentes, unto John Abbot and Covent of the seide Monasterie, and to theire successours, for any Dysmes, or any part or parcell of any Dysmes, or other Quoote* what so ever it be, to be born or paied by the Abbot and Covent of the seide Monasterie, or by theire successours, or of any of theym, or upon theym or any of theym in any wyse to be levyed, to us, or to oure heirs or successours at any tyme.
* quote: the part or share which an individual is obliged to contribute to a total amount