Short for ultimate bias. Typically used by fangirls and fanboys to describe their favourite celebrity amongst the many that they are fans of.

ymf mean's "you mother fucker".When someone make you angry you say YMF or something....

You're My Family (or Friend or Favorite)

Ymf young mental feildies also known as a gang in Scotland that get there olders to help them and use weapons and are just pussys