The sentence uses a stinky armpit as a metaphor for the park. If it were a simile, the writer would have written, "This park is really like an armpit". The metaphor makes the connection stronger. I never knew it's called an 'axilla'! But then again I'm no doctor. ;)

armpit as a slang a desirable place,interesting place as we say(who want to go to an armpit?

One variation on this is "the pits". This may be short for "armpit" but it also may be the pits of hell - nobody knows for sure. It is any awful place or situation. "I've been to to dull parties before, but this one is really the pits!" Like "armpit" it implies something very unpleasant, though armpit usually implies something messy and smelly, whereas "the pits" just suggests something unpleasant - not necessarily physical.

Hey, dear! It's litteraly "Axilla", the part under the shoulder, where the shoulder joints to the arm... When You call a place "armpit", Your just comparating the place to an armpit (Maybe "Whata an ugly place", "Whata stinky place", etc...)... HHHHH Kisses!