Principal English Translation: 

little (or, in general, a diminutive element) (See Karttunen)

Frances Karttunen: 

-PĪL diminutive compounding element; pl: -PIPĪL C gives the difficult example of PILPĪL 'little boy,’ the plural of which (to be found in M as well) is PĪPILPIPĪL, with reduplication of both the stem and the diminutive element. Since PIL 'child' has a short vowel in the stem and a long vowel in its reduplication, while diminutive -PĪL has a long vowel with short-vowel reduplication, this leads to mirrored relative vowel length (long-short, short-long) which has been confused in the 1892 reprinting of Carochi. It is correct in the original printing.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 194.

Horacio Carochi / English: 

The plural of pīl is pipīl, as with pilpīl, little boy; plural pīpilpipīl. Tōtōlpīl, [nice] little bird; plural tōtōpipīl.
Horacio Carochi, S.J., Grammar of the Mexican language with an explanation of its adverbs (1645), translated and edited with commentary by James Lockhart, UCLA Latin American Studies Volume 89 (Stanford: Stanford University Press, UCLA Latin American Center Publications, 2001), 43.