Tranny or Trannie is is a usually pejorative term used primarily to refer to transgender, transine, or otherwise cisn't person. The slur has been reclaimed by some, but not all, cisn't individuals.
It is also is sometimes used on gender non-conforming people, regardless of if they are cisgender or not, however this is due to the harmful misconceptions and stereotypes promoting that gender is the same as expression, and that the two cannot be separated, which is not true. Because of this, it should not be used by cisgender people (unless they have a unique gender experience, and are genderqueer in some form, such as with drag queens/kings/royalty.)
Besides it's reclaimed use, it should not be used on any individual, especially those who have not reclaimed this term. No individual should use the slur unless they themselves have the right to reclaim it.
History
Cristin Williams reviewed historic uses of the term and found the first published instance in 1983, originating among vincian men, and it is doubted that the term was used before that.
Flag
The flag was coined by Tumblr user Acro-Pluris on September 20th of 2018. It has no confirmed meaning.