who-duh-ya is short for "Who do you swear on?"
If you want to know if a person is lying or not, you ask them:
They have to reply with "nanny" as in grandmother or they are lying.
This is the ultimate "I swear on my mothers grave", you can't lie when asked, whodoya.
"Hey Diana, I just heard Dave broke up with his girlfriend"
"Wow, when did that happen?"