This is used when saying thank you sarcastically or when you dont really mean it but the situation deems it necessary. Its often followed by a snicker and then a good cheese. When pronounced correctly, the word will sound as though it has a rolling "A" and finishes sharply.
After seeing the .25 cent tip a customer had left her, the bartender said "Thaaanks!!"
When some one at he poker table tells you nice hand when you know you were playing rags, your reply should be "Thaaanks"
When an ugly guy or girl at the bar buys you a drink and waves when the bartender delivers it, your reply should be "Thaaanks!!"