The meaning of the Arabic word halal is ‘(that which is) allowed, permitted or permissible’. Possible transcriptions of the word include ħalāl, 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 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
( حرم )
( 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.