לֹא תִנְאָף (lo teen-AF)
Just these two words! לא, lo, as you may already know, means "no, not" When used before the imperfect verb form, as in this case, it denotes a permanent command, not just a temporary or conditional command.
תִנְאָף = tav prefix: "you will" (in this case, will NOT! because of LO just mentioned)
נאף = na'AHF: "commit idolatry/adultery, unchastity" Note the close relation between adultery and idolatry. This word means BOTH things!
Altogether "DO NOT COMMIT ADULTERY"