lm (留名 lau4 meng2) is the abbreviation of a romanised pronunciation of Hong Kong internet slang. Its direct translation is to leave (one's) name. Widely used in the comment sections of LIHKG (a multi-category forum website based in Hong Kong) to follow up/mark on a post of interest.
It is short for Lil' Momma. Her actual name is Jemima. This is a nickname she got by her friends in primary school. She's a tomboy who is obsessed and luckily dating the best boy in the world.