tlacaco. segura y pacificamente, o sin zozobras. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 115r. col. 1. Thanks to Joe Campbell for providing the transcription.
Attestations from sources in English:
tlacazo (adverb) = truly, certainly Daniel Garrison Brinton, Ancient Nahuatl Poetry: Containing the Nahuatl Text of XXVII Ancient Mexican Poems (1877), 164.