Argumentum ad dictionarium

From Speculative Catholic:
Opponent: "The dictionary defines abortion as the termination of a pregnancy. The dictionary also says that pregnancy begins when the zygote implants.

Because the morning after pill is designed to prevent implantation, it therefore does not cause abortion. You can't argue with the dictionary."

Me: "Ok, I'll concede that. But the dictionary does provides a definition that covers an effect of the morning after pill - the killing of a human by another - it's called homicide."

Opponent: [Head explodes.]