Vulgar in origin (屌 is crude slang for the male organ) but widely used self-mockingly by young men online to identify with the opposite of 高富帅. Fine in casual online speech; not for formal use.
history
Coined around 2011 on Baidu Tieba when fans of a soccer star 李毅 embraced 屌丝 as a self-label. It exploded across Chinese internet culture as shorthand for the ordinary, un-advantaged young man.