To take advantage of a woman that normally finds you extremely unattractive, usually done when thegirl is either emotionally distressed or extremely intoxicated
Bro 1: Where did audrey go? She passed out on the couch a few minutes ago
Bro 2: I think william took her upstairs....
Bro 1: There he goes again, always pulling a robbie
On December 14th, you go to a person who has been given the nickname "Robbie" and try to win him over for the day. It becomes bonus points if he is in a current relationship
"Hey, have you seduced a Robbie yet? Its still National Seduce A Robbie Day"
to mask one's shaky live performance and/or unfamiliarity with the words of one's own songs by enlisting the crowd to sing instead, often for entire verses and choruses, in the manner perfected by Robbie Williams
Bono didn't seem himself when U2 played Glastonbury: he kept doing a Robbie and getting the crowd to help him out by singing on his behalf.