A San Diego thank you is the act of kissing someone while they are taking a dump.
It is the most intimate, you can get in a human relationship.
The phrase appeared in the 15th Episode of American Dad’s 5th Season.
In one of the scenes, Roger and Stan are in a pub where Stan leaves to the toilet, then Roger sends a woman after him, telling her she should give him a “San Diego thank you”.
The episode was aired on April 18th, 2010.
The expression was defined extensively on Urban Dictionary in five different entries, all of which were uploaded on the same day; April 18th, 2010.
“San Diego thank you” would appear in several other episodes of American Dad.