Arrive, especially unexpectedly.
Or someone uninvited rocks up to your party acting like they've being there from the start, look who just blowed in.
Your playing football with u'r mates and 3 guys, nobody knows, starts playing, what blow ins.
There are three things one could do at a time like this.
(1) If you are straight and normal, you quickly remove your finger and wash it off, or
(2) if you are a demented pervert pull it out and lick it.
(3) Finally, if you are a faggot, you will continue to push it in and out.
see also, splash back