Principal English Translation: 

seed (Lockhart); plant seed (see Molina); or, when possessed, semen (see Karttunen)

Orthographic Variants: 
Alonso de Molina: 

xinachtli. semilla de ortaliza. &c.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 159r. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

XINĀCH-TLI seed / semilla de hortaliza (M) -XINĀCHYŌ necessarily possessed form one’s semen / el semen genital (C) [(3)Cf.83r]. See XINĀCH-TLI.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 325.

Lockhart’s Nahuatl as Written: 

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), 241.

Attestations from sources in English: 

calaqui yey almo tlaoli xinaxtli = into which 3 almudes of maize seed fit
(San Pablo Tepemaxalco, Toluca Valley, 1762)
Caterina Pizzigoni, ed., Testaments of Toluca (Stanford: Stanford University Press and UCLA Latin American Center Publications, 2007), 141.

ca ça yehuantin yntechpa tlatoaya in ciua in tlacaxinachtica otztia in nacayotica tlacachiuaya = for it was said only in regard to women who became pregnant with human seed, who gave birth through the body (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), 65.