To have a very small penis. Guys who are "hung like an ant" will usually be very embarassed about their little dick. Girls will mock it for being pathetic.
Simon: Lets have sex
Lizzie: Not tonight Honey
Simon: We haven't had sex in weeks.
Lizzie: Thats because I'am tired, and lets face it your hung like an ant. I have never had a real orgasim with you.