What does “creeping on one's turf” mean?

Question

When you check in on foursquare to a place whose mayor is your friend, your check-in message is "Your friend X is the mayor here (and you're creeping on their turf)."

What does this expression mean?

EDIT:

In foursquare's game mechanics, the person with most checkins (in different days) to a venue is the venue's mayor. So, when you check in somewhere you're actually getting yourself close to steal that person's mayorship.

I understand that the word "turf" here has to be a slang for territory; my main doubt is about the word "creeping," then. Does the sentence means "you're on your way to steal your friend's mayorship" or "you're enjoying the benefits of hanging out on a place that is commanded by your friend"?

Answer 1

I cannot account for any peculiarities in foursquare's behavior, but "creeping on one's turf" means something similar to "invading their territory." Typically, one's turf is akin to their:

• stomping ground
• territory
• neighborhood

Etymonline has this to say of turf:

Slang meaning "territory claimed by a gang" is attested from 1953 in Brooklyn, N.Y.; earlier it had a jive talk sense of "the street, the sidewalk" (1930s), which is attested in hobo use from 1899, and before that "the work and venue of a prostitute" (1860).

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Creep, in this case, would mean that you are approaching the turf in a manner that could be deemed a threat or an attempted takeover. This, naturally, could be exactly what was intended. From my local dictionary:

creep — (of an unwanted and negative characteristic or fact) occur or develop gradually and almost imperceptibly

With regards to turf and the corresponding turf wars, creeping would be trying to sneak in and suddenly swipe the territory away.

Answer 2

In this context creeping refers to "edging up to" or approaching. Turf is their area or property. As it's a foursquare check-in, it's not actually theirs :) But all it's saying is that you are on their property (but in this case it's said in a fun/competitive way).

Sometimes turf does not mean physical property but an area of responsibility so you may hear it in that context as well.