The Respect MP has upset many users on the social networking site after using the derogatory term for disabled people in the run-up to the Paralympic Games, which launches in London tomorrow.
Last week, Galloway was widely criticised after his remarks about the rape charges facing Wikileaks founder Julian Assange.
The MP posted the Twitter comment on Sunday when he tweeted to @Hawfa: 'you badly need medical help son. Will decent Rangers fans please substitute this windae-licker … '
It was not immediately known what had prompted the comment by Galloway, as the account run by @Hawfa - apparently hosted by a Craig Dickson, who describes himself as "Rangers morning, noon and night" - is protected and can be seen only by his accepted followers.
But Galloway's tweet was visible to his 90,000 Twitter followers on the social networking site and drew criticism from other users. Sarah Low wrote: "George Galloway being a disgrace to society again... What a disgusting little man.. Respect Party? Someone hand him a dictionary!
Dan Scorer, senior campaigns and policy manager at the learning disability charity Mencap said: "We are appalled that George Galloway, a member of parliament, has used this unacceptable term of abuse, which is deeply offensive to disabled people.
"Hate crime and bullying are a daily reality for many disabled people and the use of language like this only furthers hostility and violence. We call on him to apologise immediately."
Galloway is understood to be in Indonesia and could not be immediately contacted.
His latest tweet comes days after the MP was roundly condemned by senior figures in his own party, rape crisis groups and other MPs, after saying in a video blog last week: "Some people believe that when you go to bed with somebody, take off your clothes, and have sex with them and then fall asleep, you're already in the sex game with them.
"It might be really bad manners not to have tapped her on the shoulder and said, 'do you mind if I do it again?' It might be really sordid and bad sexual etiquette, but whatever else it is, it is not rape, or you bankrupt the term rape of all meaning."
Elsewhere today, in an unconfirmed report, Galloway is said to be joining a Lebanese TV station to present a fortnightly show, earning himself around £80,000 a year.
Al-Mayadeen, an Arabic language station which has links to the ruling regimes in Syria and Iran, portrays itself as a balance to Al Jazeera, which it believes is biased against Syria, but insists it will be politically neutral.
In one of the first episodes of his show, which has Arabic subtitles, Galloway rails against Western countries who have supported the uprising in Syria for being like "Crusaders".
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