The indispensable, even though incomplete, Arabic-English lexicon of Edward Lane is available several places online. You can download online here. Or check out http://www.tyndalearchive.com/tabs/lane/ and http://www.studyquran.co.uk/LLhome.htm

I find it unfortunate that this Victorian era lexicon, valuable as it is, has never been completed in English. Many of the meanings that Lane provides are somewhat archaic and he is also quite arbitrary in what he chooses to record. So it is always advisable to use his text more as an index and then go to Lisan al-Arab, Taj al-Arus or one of his other important sources to see what the original says. The French variant by Kazimirski is also 19th century but at least is complete. Lisan al-Arab and several other lexicons are available and searchable online at http://www.baheth.info/ There are also several earlier Arabic-English lexicons available in pdf at archive.org. But the mother of all Arabic dictionaries, al-Zabidi’s Taj al-Arus is available either online or in pdf for the valuable Kuwait edition at http://www.archive.org/details/alhelawy09