This is slurped?

Figuring out something new in a different language is fun and sometimes challenging. I have never had trouble before with French, German, Spanish, Latin, and even Greek. But Finnish is especially "fun" and "challenging." I am encountering too many words that seem to defy translation, but these words are used in everyday situations, so their meaning must be known to the five million or so speakers of Finnish. Google is great and all that (although read my post "A slap in the face for .99 euros"), but I also know very well how to use a dictionary, and I have several Suomi/Englanti dictionaries. So when I saw a seemingly simple phrase on a McDonald's poster advertising a strawberry shake, I assumed I could translate it: Tämä on ryystö. I already knew the "Tämä on" part, which is easy: This is. I was happy I could learn a new, common word that I could use with "This is" and impress everyone with my McDonald's street talk: Anteeksi, tämä on ryystö! But this is what? "Ryystö" was not the word for "shake" (that word is "pirtelö," and it is on the poster). So I figured the word must be an adjective that was informal (slang) for something like "great" or "tasty." Well, I spent way too much time (that's to say, more than five minutes) trying to discover the meaning of this word. Google Translate informed me that "Tämä on ryystö" means "This is ryystö," which I can't deny, but I know there must be some real meaning behind that word. The word sort of looked like the verb "ryystää," which means "to slurp," but it was not a conjugated form of the verb like you could expect (This is slurped?) because I checked: http://www.verbix.com/webverbix/Finnish/ryyst%C3%A4%C3%A4.html. Is it a variant (This is slurpilicious)? At least I learned a few more words trying to figure out this one, and "ryystö" is now on my list of words to ask a native speaker. The list is growing.

UPDATE: A very nice Finnish person explained this to me (Thank you, M!). The Finnish verb for "to rob" is "ryöstää," which is very close to the verb "ryystää." So it is a play on words. This is robbery (the cost is so cheap?) and this is slurp (what else do you do with a shake?). And now I know enough so that I don't use this phrase in public unless in line at McDonald's, and probably not even then.