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
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.