mapilli.

Headword: 
mapilli.
Principal English Translation: 

a finger (see Lockhart); also, a measurement, possibly 1.74 cm. (see also cemmapilli)
See: James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 224; and John E. Clark, Aztec Dimensions of Holiness, cited in: http://mathforum.org/kb/message.jspa?messageID=7615867.

IPAspelling: 
mɑhpilli
Alonso de Molina: 

mapilli. dedo dela mano.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 52v. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

MAHPIL-LI finger / dedo de la mano (M) See MĀ(I)-TL, PIL-LI.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 131.

Lockhart’s Nahuatl as Written: 

māitl, pilli.

Attestations from sources in English: 

tomapil mapiloa / tetlatitia / tlacuiloa / yc tzaoa = Our finger: It points, it indicates something to someone, it paints, with it one spins [cotton]. (central Mexico, sixteenth century)
Fray Bernardino de Sahagún, Primeros Memoriales, ed. Thelma D. Sullivan, et al. (Norman: University of Oklahoma Press, 1997), 256.

See the Matrícula de Tributos for examples of tribute cloths with fingers emerging from the tops.

Intla vtlica qujntlaҫaltia in titici, in jnacaio mocioaquetzquj: vncan imjxpan contequjlia in jmapil in tlanepantla hicac: auh intla iooaltica vel qujtataca, no qujtequjlia in jmapil, ioan qujxima, qujcujlia in jtzon = If along the road they wrested the body of the mociuaquetzqui from the midwives, in their presence they cut off her middle finger. And if they could dig her up by night, they also cut off her finger and they clipped off, they took her hair from her (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 162.

in jquac vi iaoc, inchimaltitlan caquja in tzontli, in anoҫo mapilli: injc oqujchtizque = when they went to war they inserted the hair or the finger in their shields in order to be valiant (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 6 -- Rhetoric and Moral Philosophy, No. 14, Part 7, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 162.

tilaoac cenmapilli: auh ynjc patlaoac cenmatl: auh ynjc vijac cempoalmatl = a finger thick, a fathom wide, and twenty fathoms long (central Mexico, sixteenth century)
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 2 -- The Ceremonies, no. 14, Part III, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1951), 69.

mapilli = finger;
uiac = long;
tepito = small;
tomaoac = thick;
pitzaoac = thin;
tzapatic = diminutive;
tlatzitzquia = it seizes things;
tlateteuhtzitzquia = it seizes things firmly;
tlatecoa = it accomplishes things;
tlaay = it does things;
tlatequipanoa = it works at things;
ixquich iueli [ixquich ihueli] = it is capable of all;
moch aini = doer of all;
muchi uel quichioa [mochi huel quichihua] = it can do all;
in chioaloni = [chihualoni] instrument for doing;
in tequipanoloni = instrument for working (central Mexico, sixteenth century)
mapilxocoiotl [mapilxocoyotl] = little finger
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 10 -- The People, No. 14, Part 11, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 117.

huey mapilli = thumb;
tlamapilhuiani = pointer finger;
mapilli hueyac = middle finger, longest finger;
mapilxocoyotl = little finger
Fr. Bernardino de Sahagún, Florentine Codex: General History of the Things of New Spain; Book 10 -- The People, No. 14, Part 11, eds. and transl. Arthur J. O. Anderson and Charles E. Dibble (Santa Fe and Salt Lake City: School of American Research and the University of Utah, 1961), 117.