to be very attentive to one thing without getting distracted by another thing (see Molina)
IPAspelling:
semmɑti
Alonso de Molina:
cemmati. nitla. (pret. onitlacemma.) estar muy atento a algun negocio, sin diuertir se a otro. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 16r. col. 2. Thanks to Joe Campbell for providing the transcription.
Frances Karttunen:
CEMMAT(I) vt; pret: CEMMAH ~CEMMAT to pay close attention to something; to agree about something / estar muy atento a algun negocio, sin divertirse a otro (M), están de acuerdo (Z) [(2)Zp.5,194]. Both attestations have n for m and have the vowel of the first syllable marked long, but the derivation is transparent. See CEM, MAT(I). Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 30.