To have my pants pulled down 

Other than it's literal meaning, it is metaphorically used to denote somebody being taken for a fool. Mostly used by Brits. Brought to the wider audience by Guy Ritchie's movie, Snatch.

Turkish: (looks at the caravan) Look at it. How am I suppose to run this thing from that? We'll need a proper office. I want a new one, Tommy. You're going to buy it for me.
Tommy: Why me?
Turkish: Well, you know about caravans.
Tommy: How's that?
Turkish: You spent a summer in one, which means you know more than me. And I don't want to have my pants pulled down over the price.

To have my pants pulled down by RobKnob March 6, 2012

pulled my pants down 

basically means being (shafted) or someone who has (screwed) you, ripped you off or fucked you over, usually for money.

i have just paid $200 more than i should have for that bitch, that guy has has totally pulled my pants down.

pulled my pants down by ron weasely April 27, 2006