When someone without invitation and against your will performs a full-on-tongues-n-all kiss on you but you are either too surprised/drunk/polite to push them away. The result of which is that you have effectively 'snogged' that person despite never having had any desire to do so.
Eurgh! I was just chatting to that freaky boy with the mono-brow last night when he mouth raped me. I was so shocked I didn't know what to do and before I knew it we'd already snogged.