Principal English Translation: 

something bad, very bad; something immoral

Alonso de Molina: 

Ayectli. cosa mala.
Alonso de Molina, Vocabulario en lengua mexicana y castellana, 1571, (www.idiez.org.mx), f. 3v.

Frances Karttunen: 

AHYĒC-TLI something bad / cosa mala (M) See AH-, YĒC-TLI.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 9.

Attestations from sources in English: 

Ōquitlazòtlapōlô in àcualli in àyēctli = He's (disgracefully) loved (what's) evil and immoral.
Michel Launey, An Introduction to Classical Nahuatl, translated and adapted by Christopher MacKay (Cambridge: Cambridge University Press, 2011), 220.

in ticmocuitlavitinemi in aqually, yn aiectli = you go about devoting yourselves to evil, to wickedness (central Mexico, sixteenth century)
Fray Bernardino de Sahagún, Primeros Memoriales, ed. Thelma D. Sullivan, et al. (Norman: University of Oklahoma Press, 1997), 239.

auh in yehuatl omoteneuh Maria xucoatolnamacac cenca quintzatzatzillitihuia amo huel mihtoz motenehuaz yn ixquich acualli ayectli tencuicuitlatlahtolli yc quimahuatihuia cenca quinmahuizpollotihuia canel cihuatl, auh amo ҫan icel ynin cihuatl yhuan ymon yn moteylhuiq̃ = And the said María, seller of bitter atole, went along shouting loudly at them; all the bad and filthy language with which she went scolding at them cannot be said or told. She showed great disrespect for them, for she is a woman. But it was not this woman alone; her son-in-law made the complaint along with her (central Mexico, 1613)
Annals of His Time: Don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, James Lockhart, Susan Schroeder, and Doris Namala, eds. and transl. (Stanford: Stanford University Press, 2006), 252–3.