In the UK some Chinese use a word pronounced like 'a cha' (my guess in pinyin) to describe South Asian people (generally regarded as people of Indian heritage but could include Pakistanis, Bangladeshis, Sri Lankans, etc.). I have heard this word used by both Chinese immigrants of Hong Kong origin and by newer mandarin-speaking Chinese immigrants from the mainland but have been told that this word has a Cantonese language origin.
The word seems to have a vulgar connotation. I have been told a vague etymology that the word relates to the perceived eating habits of South Asians, that they use their hands to eat rather than forks, but I've no idea if this is true.
Can anyone clarify the origin of this word and give a written form (if existing)?