Technically what is a FD? A safe bet which does not go down in value of principal or interest even when economy fluctuates.
So thats what clearly they meant Aarti is. Someone who will stay by Sid's side no matter what. There was nothing dirty about it. Aarti has time and again proved this is true and correct by always being Sid's tail in all decisions and votes. What it meant is she is another safe solid vote for Sidharth.
Trust the dirty minds like Shefali and Sid to try and concoct a meaningless "dirty" aspect to this.
Of course being someone's fd in a game like BB is also not a good thing but then so are Pahira to each other. Thats how some ppl (especially where one is weak) play the game.
So IMO... It is a ekdum correct statement for Aarti, not flattering --- but certainly doesnt have any dirty meaning as they are trying to imply.