Halal and haram

The meaning of the Arabic word ‘halal’ is ‘(that which is) allowed, permitted or permissible’. Possible transcriptions of the word include ħalâl, ḥalāl and ħala:l.

In Arabic it is spelled حلال. The root of the word is h-l-l, or حلّ in Arabic script. That verb has meanings like to untie, unbind, unfasten, unravel, solve, decipher.
Source: "A Dictionary of Modern Written Arabic, by Hans Wehr, edited by J M. Cowan, 1976.
The meaning of the adjective is probably connected with derived verb stem IV, 'aktaba (source: Teach yourself Arabic, A.S. Tritton, 1958), which among other things can mean "to declare lawful, legally permissible, allow".

The opposite of halal is haram, for which in the dictionary I find both ħaram (حرم) and ħara:m (حرام).
ħaram means forbidden, prohibited, but also taboo, holy, sacred, sacrosanct. ħara:m means forbidden, prohibited, unlawful; offense, sin; inviolable, taboo; sacred, sacrosanct. The root and verb from which it derives, ħaruma (حرم) means to be forbidden, unlawful, etc.