xi-.

Headword: 
xi-.
Principal English Translation: 

command form (optative, imperative)

Lockhart’s Nahuatl as Written: 

x(i)-. replaces the 2nd person subject prefixes of verbs in the optative/imperative.
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: 

ximotlāli = sit down (tlālia);
xinēchpalēhui = help me (palēhuia);
xitlàto = speak (ìtoa);
xictēmo = look for it (tēmoa)
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 78.

Attestations from sources in Spanish: 

cuix noço ytlah yhtlacauhtica melahuac xiquihtocan = ¿o acaso está distorsionada la verdad? digan. (Tlaxcala, 1609)
Vidas y bienes olvidados: Testamentos en náhuatl y castellano del siglo XVII, vol. 3, Teresa Rojas Rabiela, et al, eds. (México: CIESAS, 2002), 64–65.