Terry Sweeney knows how to get under Nancy Reagan's skin. After all, he made his name doing impersonations of the former first lady on Saturday Night Live. Five years later he's still turning her to his own account. Perfectly coiffed, and clad in the familiar red Adolfo suit, Sweeney peers archly from the cover of It's Still My Turn: A Satirical Collection of Nancy Reaganesque Poetry (published by ''and books''). ''My book used to be called An Unauthorized Collection of Nancy Reagan Poetry, but the lawyers had a fit,'' Sweeney confides. ''We had to give it an unsuable title.'' His version of Nancy's opinions may be slightly more litigious. Victims include Dan Quayle (''He's young and he's handsome/and so full of life/It's a shame that he's stuck/with that feminist wife''), Raisa Gorbachev, Alexander Haig, Maureen Reagan, Imelda Marcos (''The crimes of Imelda Marcos/are still a mystery to me/All that poor woman did/was go on a shopping spree''), and Kitty Kelley. Sweeney says he's looking forward to Kelley's forthcoming tell-all book on Nancy Reagan. ''Kitty has a way of finding out everything,'' he says. ''I think the Adolfo suit's going to be hitting the fan.''