tlein.

Headword: 
tlein.
Principal English Translation: 

what? (interrogative), but when preceded by in, that which (see Lockhart, Karttunen, and Molina)

Orthographic Variants: 
tleyn, achtlein
IPAspelling: 
tɬein
Alonso de Molina: 

Tlein? que?
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, f. 147r.

Frances Karttunen: 

TLEIN what? / ¿qué? (M), ¿qué?, o ¿qué cosa? (C) Although M gives only an interrogative sense, there are abundant attestations in which TLEIN has a non-interrogative, relative sense. This has shortened to TLEN in Z and in many other Nahuatl speech communities. See TLE(H), IN.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 308.

Lockhart’s Nahuatl as Written: 

what, interrogative; preceded by in, that which. tle(h), in.
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), 239.

Attestations from sources in English: 

something, what
Beyond the Codices, eds. Arthur J.O. Anderson, Frances Berdan, and James Lockhart (Los Angeles: UCLA Latin American Center, 1976), 19.

tlein tlahuilli, tlein tlanexpepetlaquillotl tlein tlatlaçotlaliztetl. ic tlatlaya = what illumination, what shimmering light, what fire of love was burning (early seventeenth century, Central Mexico)
Louise M. Burkhart, Before Guadalupe: The Virgin Mary in Early Colonial Nahuatl Literature, Institute for Mesoamerican Studies Monograph 13 (Albany: University at Albany, 2001), 57.

in tlein = how
Rebecca Horn's notes from Nahuatl classes with James Lockhart, currently being harvested for this dictionary by Stephanie Wood.